NGC 2818 is an intermediate age open star cluster that is located in the southern constellation Pyxis (also known as The Compass). Within NGC 2818 lies a planetary nebula. Since open clusters usually disperse over hundreds of millions of years, it is very rare to find an open cluster that is actually old enough for one of its stars to form a planetary nebula. NGC 2818 is also believed to be about 1 billion years old. Therefore, NGC 2818 is quite special.
In a recent study, Sharmila Rani, Gajendra Pandey, Annapurni Subramaniam, and N. Kameswara Rao (2023) identified four hot and bright blue straggler stars within NGC 2818 by using colour-magnitude diagrams and the first far-UV images of the cluster. By using spectral energy distributions, they estimated the parameters of the blue straggler stars and found that these stars were either collisional products or that they might have undetectable companion white dwarf stars. In their study, they also identified two yellow straggler stars, which they confirmed to be binary based on their photometric analyses of these stars. They also found no evidence to suggest that the planetary nebula isn’t part of NGC 2818. They determined that the planetary nebula’s central star was in its white dwarf phase and that it currently had a mass of about 0.66 solar mass and that the progenitor star’s mass was about 2.1 solar mass. This suggests that it’s possible for the progenitor star to have formed as part of the cluster since its mass fits with the turn-off mass of the cluster.
In order to study NGC 2818 and the relationships between its colours, brightnesses, age, and chemical composition, I collected 15 images in total using the Prompt5 and the PROMPT-MO-1 telescopes by using Skynet. 5 of the 15 images were taken with a B filter, another 5 images were taken with a V filter, and the last 5 were taken with an R filter. The reason for taking images in all three of these filters is so that I would be able to make a full colour image of NGC 2818 by using Afterglow. Of the 5 B filter images, 4 were taken with the Prompt5 telescope with an exposure length of 146.34 seconds each and the other 1 image was taken with the PROMPT-MO-1 telescope with an exposure length of 153.85 seconds. For the 5 images taken with a V filter, 4 images were taken with the Prompt5 telescope with an exposure length of 60.98 seconds each and the other 1 image was taken with the PROMPT-MO-1 telescope with an exposure length of 64.1 seconds. For the 5 images taken with an R filter, 4 images were taken with the Prompt5 telescope with an exposure length of 48.78 seconds each and 1 image was taken with the PROMPT-MO-1 telescope with an exposure length of 51.28 seconds. To set up these images in Skynet, I set the Max Sun Elevation to -15, the Min Target Elevation to 30, the Max Moon Phase to 50, and the Min Moon Separation to 60 while all other options were left as the default values. All of these images were also taken with Generic 16-inch telescope efficiency.
After I got my images back on Skynet, I opened them on Afterglow and then batch photometered my images. I then saved the batch photometry data and uploaded it to Cluster Pro Plus. In Cluster Pro Plus, I found my cluster’s Motion in RA to be -4.401 ± 0.647 mas/yr, its Motion in Dec to be 4.585 ± 0.759 mas/yr, and the distance to the cluster to be 3.59 kpc ± 22.39%. Then, I downloaded the Gaia and 2MASS data for my cluster after which I determined the values of NGC 2818’s log(age), metallicity, and E(B-V) by fitting the isochrone model as best as I could to both a graph of Gaia M_RP vs BP_RP and a graph of 2MASS M_H vs J_H, whilst focusing more on the graph containing the Gaia data. My Gaia and 2MASS graphs looked as follows:
Image 1: Gaia and 2MASS graphs
By fitting the isochrone model to these graphs, I found the log(age) to be 8.6 log(yr) (which corresponds to a calculated age of 398107170.553 years), the metallicity to be -0.26 solar, and the E(B-V) value to be 0.23 mag. After I fitted the isochrone models to the Gaia and 2MASS data, I went back to Afterglow and made two colour images, an ISM-reddened image and an ISM-de-reddened image. I got the ISM-reddened image by setting the E(B-V) value to zero and I got the ISM-de-reddened image by setting the E(B-V) value to 0.23 mag. Below are my ISM-reddened and ISM-de-reddened images:
 
Image 2: ISM-reddened image of NGC 2818
Image 3: ISM-de-reddened image of NGC 2818
After I made the colour images, I refitted the isochrone model to the Gaia and 2MASS graphs by using the parameters found in the MWSC catalog. The resulting isochrone model graphs are below:
Image 4: Gaia and 2MASS graphs with MWSC parameters
The MWSC catalog listed the cluster’s log(age) as 9.16 log(yr), the metallicity as -0.17 solar, and the E(B-V) value as 0.115 mag. I did have trouble fitting the isochrone model to both the Gaia and the 2MASS data with these parameters, which is either because I made a mistake when I fitted the isochrone model or because there is something wrong with the data. By comparing my estimations of the log(age), metallicity, and E(B-V) values to the MWSC catalog’s values, I found that my values were probably wrong since the graphs using the MWSC values seemed to have a better isochrone model fit (especially in the case of the Gaia data graph) than the graphs with my estimated values did. I therefore concluded that my estimations don’t really improve on previously published data, since the previously published data seem more accurate.
Overall, this assignment was a lot of fun. I especially liked making the colour images of the different star clusters and it was very interesting to try and find the values of the different parameters by fitting the isochrone model to the data.