Astronomical and Imaging Data

Useful Informations

Terzan 6 is a $\star$ heavily obscured globular cluster located in the Galactic Bulge of the Milky Way, characterized by its intermediate metallicity and its status as a host for highly energetic compact binary systems.

---

Galactic Location and Extinction

Terzan 6 is found in the constellation Scorpius. Its location in the dense inner region of the Galaxy means it suffers from significant interstellar extinction (reddening), which makes observing its visible light challenging.

• Distance: The cluster is relatively close to the Galactic Center. Recent, detailed studies estimate its distance from Earth to be approximately 6.8 kpc (about 25,500 light-years). Its projected distance from the Galactic Center is estimated to be around 1.3 kpc.
• Obscuration: It is subjected to a very high level of dimming and reddening from intervening dust, with a color excess measured as high as E(B-V) 2.36 magnitudes.

---

Stellar Properties and Age

Terzan 6 is a typical, ancient globular cluster, providing insights into the early formation of the Galactic Bulge.

• Age: It is an old system, with an estimated absolute age of approximately 13 Gyr, consistent with the ages of other old globular clusters found in the Galactic Bulge.
• Metallicity: Spectroscopic analysis classifies it as an intermediate-metallicity cluster, with a mean iron abundance of [Fe/H] -0.65 dex.
• Chemical Abundances: Like other metal-rich bulge clusters, its member stars show a notable α-element enhancement (e.g., enhanced concentrations of elements like O, Mg, and Si relative to Fe), which is a chemical signature indicative of a rapid star formation phase dominated by core-collapse (Type II) supernovae in its past.
• Multiple Populations: Slight variations in the abundances of light elements suggest the presence of multiple stellar populations (first- and second-generation stars), a feature commonly found in massive, old globular clusters.

---

Dynamical State and X-ray Emission

The high stellar density in Terzan 6 has led to the formation of rare, energetic binary systems.

• Dynamical Evolution: Analysis of its density profile indicates that Terzan 6 is in an advanced stage of dynamical evolution, with a high concentration parameter, suggesting it is either a very concentrated cluster or a potential post-core-collapse system, where the central region has contracted to a high stellar density.
• X-ray Binaries: Terzan 6 is a host to two low-mass X-ray binaries (LMXBs), which are compact star systems that emit high-energy X-rays. The most famous is the recurrent X-ray transient GRS 1747-312, a system containing a neutron star rapidly accreting matter from a normal companion star, causing bright Type-I X-ray bursts. The presence of multiple LMXBs is a direct consequence of the high stellar encounter rate in the dense core of the cluster.

---