NGC 3201, (also known as Caldwell 79) is an old, low galactic latitude globular cluster in the southern constellation of Vela and is considered to have a relatively very low concentration of stars. NGC 3201 was discovered by James Dunlop on May 28, 1826. He is noted to quote the cluster as “a pretty large pretty bright round nebula, 4′ or 5′ diameter, very gradually condensed towards the centre, easily resolved into stars; the figure is rather irregular, and the stars are considerably scattered on the south.”
The cluster has a radial velocity of 490km/s which is higher than any other cluster known. NGC 3201 has an age of approximately 10.24 billion years and is located at a distance of 16,300 light years from the Sun. In addition to this, the cluster is an estimated 157,000 times the mass of the Sun.
In a recent study, conducted by Benjamin Giesers and a team of astronomers in 2018, they’ve found reason to believe there is a detached stellar-mass black hole located in the cluster. While spectroscopic surveying 25 Galactic globular clusters with
MUSE, Geisers and his team performed multiple epoch observations of NGC 3201 with the aim of constraining the binary fraction. From this investigation into NGC 3201 they speculate a detached stellar-mass black hole in the cluster stating:
“The data show strong evidence that the target star is in a binary system with a non-luminous object having a minimum
mass of (4.36 ± 0.41) M. This object should be degenerate, since it is invisible and the minimum mass is significantly
higher than the Chandrasekhar limit (∼ 1.4 M). The small distance of the system from the centre of the globular cluster (10.800) is expected from mass segregation. Most likely, the degenerate object has exceeded the Tolman-Oppenheimer Volkoff limit that predicts all objects to collapse into black holes above ∼ 3 M (Bombaci 1996).” (Giesers et al., 2018)
The article goes on to state that a a double neutron star system with both components having more than 2.0 M could explain the observations, but since such a star system has not yet been observed, a situation involving black holes is concluded to be much more likely.
This study proves beneficial as it provides additional context to NGC 3201’s unusual structural core radius, as reinforced by Giesers and his team the presence of black holes can lead to an expansion of the core radius through interactions between black holes and stars.
In order to study NGC 3201 and investigate the relationships between the colours, brightnesses, and the age and chemical composition of the cluster as a whole, I collected a total of 15 images using Skynet telescopes. Five 30 second images were taken with a B filter, five 15 second images were taken with a V filter, and five 15 second images were taken with an R filter. After collecting the images, I utilized Afterglow and Cluster Astromancer for data analysis, including generating isochrone models, color images of the cluster, and HR diagrams.
Through this, I found the following data:
Classification: Globular
Star Count: 3307
Mass: 266704.3631
Physical Radius: 13.3633
RA: 154.4
DEC: – 46.42
And with further refinements from the MWSC catalogue:
E-BV: 0.239
Log age: 10.1
Inevitably modelling them under the following models:
Isochrome Model (H vs J-H)
Isochrome Model (RP vs BP-RP)
Histogram
As part of my Astronomy 113 coursework, I conducted this analysis with the aim of enhancing my comprehension into star clusters, their characteristics, and behaviours. Upon further investigation, It’s become very apparent NGC 3201’s unique characteristics such as it’s low concentration of stars, old age of the system, and straggling stars that fit outside the mold of conventional models.
Comparing my data to the data in MWSC, it was pretty accurate aside from my estimated age and E-BV figures which perfected the Isochrone models and reinforced the MWSC’s calculations for this cluster.
Having completed my analysis, I have developed a deeper understanding into how to photometer, colour, and calculate factors such as age and reddening for star clusters. The most impactful portion was understanding the different behaviours and influences to a variety of clusters, such as blue stragglers and horizontal branches.
References:
https://arxiv.org/pdf/1801.05642.pdf
https://en.wikipedia.org/wiki/NGC_3201