TRUMPLER 23 (AKA COLLINDER 295) & RCW 99

OPEN CLUSTER AND HII REGION IN NORMA (THE SQUARE RULE)

(Image centered at: ra 16 h:00 m / dec - 53º 31')

October - 2024, Home Backyard in Martinez, Buenos Aires, Argentina

DATA

TYPE: OPEN CLUSTER & HII REGION

APPARENT DIAMETER: 9 arc seconds 

APPARENT MAGNITUDE (V): 11.2

DISTANCE: 7100 light years (2200 parsecs)

IMAGE INFORMATION

INSTRUMENT: 6" ORION OPTICS UK (Ultra Grade Optics) w/Sky Watcher Coma Corrector (0.9x) working at at f4.5

CAMERA: QHY 183 MONO

MOUNT: VIXEN GDPX, OAG with Starlight Xpress Lodestar

FILTERS: BAADER LRGB Set ANTLIA & HA

SKY CONDITIONS: urban skies - Bortle 8

EXPOSURES: LHARGB (45,60,45,45,45)

OBJECT DESCRIPTION AND IMAGE SESSION

Trumpler 23 is a moderately populated, intermediate-age open cluster within the solar circle at a RGC ~ 6 kpc. It is in a crowded field very close to the Galactic plane and the color-magnitude diagram shows significant field contamination and possible differential reddening; it is a relatively understudied cluster for these reasons, but its location makes it a key object for determining Galactic abundance distributions. New data from the Gaia-ESO Survey enable the first ever radial velocity and spectroscopic metallicity measurements with the possibility to determine 167 potential members.

Observer Magda Streicher decribed Trumpler 23 as a "..large, faint gathering of about 30 stars in an elongated northwest to southeast direction. Resembles to me an image of a thick number 2. The resemblance is unique. Brighter stars made themselves known randomly over the surface of the cluster. Although faint, it still standing out towards a beautiful pinpoint star-field. A beautiful faint yellow star can be seen on the east periphery" With a bit of imagination de "bold 2" can be distinguished in the image above.

The bright nebula at the lower right is RCW 99. According to a 1975 paper, RCW 99 is an extended HII region with low density and 102 solar masses of gas and dust. Avedisova lists 5 ionizing stars, including the hot O5 III giant LSS 3443, the O9.5 II giant CP -53 6950 and 3 B-class super giants. She places the nebula in star formation region SFR 328.60-0.51 along with the star cluster Trumpler 23. The Georgelins give an O7 III class for LSS 3443 and SIMBAD lists it as a B-class star.

A 2006 paper gives an age of 700 million years and a distance of 2200 parsecs for Trumpler 23, raising some doubts as to whether this cluster can really be associated with RCW 99.