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A new spectroscopic investigation of Terzan 9 using MUSE RSS feed
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Joined: 17/10/2019
Messages: 96
Location: Mount Kuring-Gai NSW Australia
Deep sky enthusiast, Barbara Wilson, who sadly passed away recently, described the visually challenging, 16th magnitude globular cluster Terzan 9, in Sagittarius, as follows :-

Originally Posted by Barbara Wilson wrote:
It is really there. The 11 mm Nagler did not quite pull it out of the background sky but the 9 mm did. Extremely weak object just at the plotted position. The cluster is very close (3') to a very elongated right triangle of 3 stars about 9-10th magnitude. Two of the three stars point to a faint star not plotted on MegaStar, of about 16th magnitude. The globular is just to the west of the faint star. The cluster itself appears visually larger that on the POSS print. The glow is maybe 1' in size. The cluster is just on the edge of a dark dust lane in the Milky Way as the CCD image shows.

Now astronomers using the Multi Unit Spectroscopic Explorer (MUSE) have been able to make observations of Terzan 9 to better understand its chemical composition.

Tomasz Nowakowski at Phys.org reports today :-

Originally Posted by Tomasz Nowakowski , Phys.org wrote:
Located only 2,280 light-years away from the galactic center, Terzan 9 is a very compact and moderately metal-poor globular cluster. Observations show that the cluster remains confined within about 3,260 light-years of the galactic center with an orbit co-rotating with the Milky Way's bar.

However, although many studies of Terzan 9 have been conducted to determine its fundamental properties, its chemical composition still remains poorly understood. In order to change this, a group of astronomers led by Heitor Ernandes of the University of São Paulo, Brazil, employed the MUSE instrument at ESO's Very Large Telescope (VLT) in Chile to conduct detailed observations of this cluster.

"Given its compactness, Terzan 9 was observed using the Multi Unit Spectroscopic Explorer at the Very Large Telescope. The extraction of spectra from several hundreds of individual stars allowed us to derive their radial velocities, metallicities, and [Mg/Fe]," the astronomers wrote in the paper.

In general, MUSE observations allowed the team to obtain spectra of over 600 stars. This sample was then reduced to 67 member stars of Terzan 9. As noted in the paper, the study resulted in measuring such properties of member stars as radial velocities, metallicities and magnesium-to-iron abundance ratio, which also gave mean values for the cluster.

When it comes to the chemical composition of Terzan 9, the observations found that it has a metallicity of approximately -1.1 and a magnesium-to-iron abundance ratio at a level of about 0.27. The metallicity is consistent with previous studies pointing out to a value between -2.0 and -0.99.

The mean heliocentric radial velocity of Terzan 9 was calculated to be 58.1 km/s, which is lower than the value from derived by a previous study based on six stars. However, the astronomers noted that both results are in agreement within uncertainties.

The researchers concluded that the results make Terzan 9 a moderately metal-poor blue horizontal branch cluster like HP 1, NGC 6558, and NGC 6522. Moreover, the magnesium-to-iron abundance ratio suggest that the stars in this cluster were formed from gas resulting from an early fast chemical enrichment by core-collapse supernovae.

Full story here :-

Paper published at arXiv 22 Oct 2019, "A MUSE study of the inner bulge globular cluster Terzan 9: a fossil record in the Galaxy" by H. Ernandes et. al. PDF (free) :-

Gary Kopff
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