Terzan 5

Physical properties

The absolute magnitude of Terzan 5 is at least . The small core of Terzan 5—about 0.5 pc in size—has one of the highest star densities in the galaxy. Its volume mass density exceeds 106 /pc3, while its volume luminosity density exceeds 105.5 /pc3, where and are the Sun's mass and luminosity, respectively. The cluster also has one of the highest metallicities among the Milky Way's globular clusters—[Fe/H]=−0.21.

In 2009 it was discovered that Terzan 5 consists of at least two generations of stars with ages of 12 and 4.5 billion years and slightly different metallicities, possibly indicating that it is the core of a disrupted dwarf galaxy, not a true globular cluster. The cluster also contains around 1300 core helium burning horizontal branch (HB) stars, including at least one RR Lyrae variable star.

Pulsars and X-ray sources

Terzan 5 is known to contain 49 millisecond radio pulsars as of December 2023, the largest MSP population among all globular clusters in the Galaxy; their true number may be as high as 200. The first such object, PSR B1744-24A, discovered in 1990, has the period of 11.56 ms. The population of pulsars inside Terzan 5 includes PSR J1748–2446ad, the fastest known millisecond pulsar, which is spinning at 716 Hz (the rotation period is 1.40 ms).

Terzan 5 also contains an X-ray burster, discovered in 1980, known as Terzan 5 or XB 1745-25. It also contains around 50 weaker X-ray sources, many of which are likely Low-mass X-ray binaries (LMXB) or cataclysmic variables.

The large number of X-ray sources and millisecond pulsars may be a direct consequence of the high density of the cluster's core, which leads to a high rate of star collisions, and to formation of close binaries, including binary systems which contain a neutron star.

In addition to discrete X-ray sources Terzan 5 produces a diffuse non-thermal X-ray emission and high (a few GeV) and ultra-high (0.5–24 TeV) energy gamma-rays. The high energy gamma rays probably originate in the magnetosphere of abundant millisecond pulsars, while ultra-high energy gamma rays likely result from the inverse Compton scattering by the relativistic electron emitted by the pulsars off the cosmic microwave background radiation.

Gallery

File:Commotion in a Crowded Cluster.jpg|Hubble image of the ancient globular cluster Terzan 5 File:The unusual cluster Terzan 5 (eso1630a).jpg|Terzan 5 is one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way.

Notes

References

References

1. "Cl Terzan 5".
2. (2010). "New Density Profile and Structural Parameters of the Complex Stellar System Terzan 5". The Astrophysical Journal.
3. (2021). "A new class of fossil fragments from the hierarchical assembly of the Galactic bulge". Nature Astronomy.
4. "Pulsars in Globular Clusters".
5. (2011). "Very-high-energy gamma-ray emission from the direction of the Galactic globular cluster Terzan 5". Astronomy & Astrophysics.
6. "The unusual cluster Terzan 5".
7. King, I.R.. (1972). "The identity and aliases of the stellar system Terzan 5". Astronomy and Astrophysics.
8. Racine, René. (1975). "UBV photometry of faint globular clusters". The Astronomical Journal.
9. (1981). "Discovery of two new burst sources in the globular clusters Terzan 1 and Terzan 5". The Astrophysical Journal.
10. (2006). "A Radio Pulsar Spinning at 716 Hz". Science.
11. (2007). "Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry". Astronomy and Astrophysics.
12. (1996). "NTT VI photometry of the metal rich and obscured bulge globular cluster Terzan 5". Astronomy and Astrophysics.
13. (2009). "The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge". Nature.
14. (2002). "Hubble Space Telescope/NICMOS observations of Terzan 5: stellar content and structure of the core". The Astrophysical Journal.
15. (2006). "Faint X-ray sources in the globular cluster Terzan 5". The Astrophysical Journal.