Introduction
NGC4609, alternatively recognized as Caldwell 98, is a young open cluster within the southern constellation of Crux – a cross-shaped asterism often referred to as the Southern Cross. A fascinating fact about the Crux constellation is that it is widely used as a national and distinctive symbol in many countries (Wikipedia contributors, 2024).
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| Figure 1.1: NGC 4609 in Crux |
As the name suggests, Crux as a constellation and NGC 4609 specifically are best observed in the southern hemisphere because they lie significantly to the south. Therefore, the cluster is not observable from our city (Saskatoon) because it never ascends above the horizon.
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| Figure 1.2: Crux in nationalflags and insignia |
The cluster is well situated behind the dark cloud known as the Coalsack Nebula, according to Rodgers(1960)[1]. He identified that NGC4609 is 1520 pcs away while the Coalsack was 174 pcs. Unlike its famous constellation that is well-known to many cultures, NGC 4609 has very few studies and research on it. The first paper on NGC 4609 was released in 1971 by Feinstein & Marraco [1], in which they identified 33 genuine members of this cluster and provided that the cluster was 60Myr in age. In the most recent study by Drazdauskas et al.[2], they updated the cluster’s chemical compositions, such as the solar metallicity of [Fe/H] = 0.16 +- 0.08, a turn-off mass of about 5.6M(sun) and a Galactocentric distance of 7.5 kpcs.
Observations
Since the cluster has so little work, I decided to study this particular cluster. To observe NGC4609, I first set up my observation through the robotic telescope Skynet. I requested five images for each filter B, V, and R from the PROMPT-5 telescope in Cerro Tololo. My maximum sun elevation, minimum target elevation, minimum moon separation, and maximum moon phase were set at -18, 30, 70, and 60, respectively. I then put the number of exposures to 1 for each filter and the image duration to 30, 20, and 15 for BVR. Figure 2 is the B, V ,R, and the photometer image of NGC4609 using AfterGlow.
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| Figure 2: B, V, R and Photometry Image of NGC 4609 (left-to-right) |
The photometric data is then uploaded to Clustermancer by Skynet for evaluation and Isochrone Matching. Before Isochrone matching, I remove any field stars and add stars from catalogs like GAIA, APASS, etc. Following up, I created V vs B-V, R vs B-R, RP vs BP-RP, and H vs J-H plots to refine their metrics. Below in Figure 3, the RP vs BP-RP that I created using Clustermancer.
| Name | NGC4609 |
| Classification | OPEN |
| Star Count | 246 |
| Mass | 1102.64329700153 |
| physical_radius | 4.48993186891883 |
| ra | 190.59 |
| dec | -62.99 |
| I | 121.902666664749 |
| b | -0.13656482199417 |
| angular_raidus | 0.0710581631298434 |
| pmr_ra | -4.89 |
| pm_dec | -1 |
| velocity_dispersion | 0.111546763248689 |
| distance | 1.11 |
| log_age | 7.9 |
| metallicity | -0.7 |
| e(b-v) | 0.41 |
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| Figure 3: RP vs BP-RP HR Diagram of NGC4609 |
From fine-tuning the parameters, the result I concluded is presented in Table 1. NGC 4609 has a Proper Motion Right Ascension of -4.89(mas/yr) and a Proper Motion Declination of -1 (mas/yr). These metrics are also shown in Figure 2. Moreover, the distance from the cluster is 1.11 (kpc), log age of 7.9 log(yrs), metallicity -0.7(solar), and reddening 0.41(mag). Figure 2 is the image of NGC4609 with the de-reddening value from my analysis.
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| Figure 3: Reddening Image vs De-Reddening Image | |
In addition, the Isochrome model reveals that the cluster is still very young, as its brightest stars are only at the turn-off point. There are three points close to the turn-off point that could be blue stragglers. From the analysis, I also realized that some stars in NGC 4609 are spread around its main sequence stars. Moreover, another paper by Feinstein and Marraco [3] suggests that there are stars from Hogg15 that can mix in with NGC 4609, hence not all the stars in this diagram are guaranteed to belong to NGC4609.
Although the project was exciting and improved my understanding of stellar evolution, my findings are similar to the previously published value of NGC4609. Yet, understanding the stars properties through this project was fun since there has not been a lot of work done on it. Overall, I enjoy the project a lot and it excites me in my future projects in astronomy.
References
- Feinstein, A., and H. G. Marraco. “THE OPEN CLUSTER NGC 4609 BEHIND THE COALSACK.” Publ. Astron. Soc. Pac. 83: 218-21(Apr 1971) 83.492 (1971): 218–221. Print.
- Drazdauskas, Arnas et al. “Chemical Composition of Evolved Stars in the Young Open Clusters NGC 4609 and NGC 5316.” Monthly notices of the Royal Astronomical Society 462.1 (2016): 794–803. Print.
- Feinstein, A., and H. G. Marraco. “Stars near the Open Cluster HOGG 15 and Not Members of NGC 4609.” Publications of the Astronomical Society of the Pacific 83 (1971): 219. Print.
- NGC 4609 (Coalsack Cluster) – open cluster in Crux. TheSkyLive.com. (n.d.). https://theskylive.com/sky/deepsky/ngc4609-coalsack-cluster-object
- Wikipedia contributors. (2024, March 29). Crux. Wikipedia. https://en.wikipedia.org/wiki/Crux