Astronomical and Imaging Data

Useful Informations

NGC 6652 (also designated GCl 98) is a compact, metal-rich globular cluster located in the constellation Sagittarius. It is a scientifically significant object due to its position in the transition zone between the Galactic bulge and the inner halo, as well as its remarkably high concentration of exotic high-energy stellar systems.

Galactic Context and Location

NGC 6652 is situated approximately 30,000 light-years (9.2 kpc) from Earth and only 6,500 light-years (2.0 kpc) from the Galactic Center.

• Classification: While often physically located within the boundaries of the Galactic bulge, it is frequently classified as an inner halo cluster that has been “trapped” by the gravitational potential of the Milky Way’s bar.
• Reddening: Like many bulge-proximate clusters, it suffers from significant interstellar reddening (E(V-I) = 0.15), though it is less obscured than clusters lying directly in the Galactic plane.

Age and Chemical Composition

NGC 6652 is one of the oldest known structures in the Milky Way, with an estimated age of 11.7 to 13.6 billion years.

• Metallicity ([Fe/H]): It is relatively metal-rich for such an ancient cluster, with an iron abundance measured at approximately -0.81 to -0.96 dex.
• Chemical Evolution: Its high metallicity at such an advanced age suggests it formed from gas that was rapidly enriched by early supernovae in the dense inner regions of the Galaxy. Modern spectroscopic surveys (such as APOGEE) have identified multiple stellar populations within the cluster, evidenced by varying abundances of nitrogen, carbon, and oxygen, indicating at least two distinct episodes of star formation.

Structural Dynamics and Core Collapse

The cluster is characterized by an extremely high central concentration, classified as Shapley-Sawyer Class VI.

• Dynamic State: NGC 6652 is often considered a post-core-collapse candidate. This means the stars in its center have become so densely packed that they behave as a “fluid,” where gravitational interactions frequently lead to the formation of tight binary systems and exotic stellar remnants.
• Blue Stragglers: The core contains a prominent population of Blue Stragglers—stars that appear younger and bluer than they should be, likely formed through direct stellar collisions or mass transfer between binary companions in the crowded central environment.

High-Energy Sources and Pulsar Population

Despite its relatively low total mass, NGC 6652 is exceptionally efficient at producing high-energy sources:

• XB 1832-330 (Source A): This is the brightest X-ray source in the cluster. It is an ultracompact low-mass X-ray binary (LMXB) consisting of a neutron star accreting matter from a degenerate companion. The orbital period is estimated to be extremely short, potentially under an hour.
• NGC 6652B: This source is a strong candidate for a transitional millisecond pulsar (tMSP). These rare systems switch between a rotation-powered pulsar state (emitting radio waves) and an accretion-powered X-ray binary state.
• PSR J1835-3259A: A millisecond pulsar with a spin period of 3.89 ms. It is notable for its high orbital eccentricity (e = 0.95), which is unusual for a recycled pulsar and suggests it has undergone significant dynamical interactions with other stars in the cluster core.

Scientific Significance

NGC 6652 serves as a critical laboratory for studying stellar dynamics in high-density environments. Because it contains both a confirmed ultracompact binary and a transitional pulsar candidate, it provides unique data for theoretical models of how binary systems evolve and how neutron stars are “recycled” into millisecond pulsars. Furthermore, its chemistry provides a window into the formation history of the Milky Way’s inner halo and its interaction with the central bulge.

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