RCW 71 (Gum 46) nebula [1] is located behind the Coalsack dark nebula at a distance of about 6.8 kly [2, 3] and thus getting significantly reddened due to light absorption by that dark nebula. The bulk of Coalsack nebula sits at a much closer distance of about 650 ly with the secondary dark clouds located as far as 10 kly in its corners [4, 5] but not where the RCW 71 is situated. Thus, RCW 71 gets most of the attenuation from that bulk of the Coalsack nebula and thus is easily detectable even on wide angle images of the area such as [6] as a bright red spot of about 2 arc. min in diameter. At the measured distance of 6.8 kly this angular diameter corresponds to the linear size of about 4 ly for the core (the HII region) of RCW 71.

Earlier observations of this object by Midcourse Space Experiment (MSX) [7] revealed that it is surrounded by a ring of infrared emissions in the MSX A (8 mkm) spectral band [2, 3] for which the optical counterpart in Ha is revealed for the first time in this new image extending as far as 6 arc. min from the central star (HD 311999). Many additional details can be seen here in the core of the nebula such as two incomplete circles of dark nebulae filaments and what appears to be an ionization shock front about 1 arc.min away from the central star (above it in the specific orientation of the image). Another interesting feature revealed in the halo is what appears to be a thick dark filament stretching from the core of RCW 71 into the top right corner of the image which could be at least partially a denser part of Coalsack nebula overlapping with more distant structures.

Unfortunately, the RCW 71 was just left out of the survey bands of Spitzer IRAC Equatorial Survey [8, 9] so further cross-referencing of the fine details of the optical counterpart of the halo in near-IR is difficult. However Herschel targeted this object over several observation sessions and produced details narrow FOV [10] and wide angle FOV [11] views of it in 100-160 mkm and 250-500 mkm spectral bands correspondingly that do confirm the presence of the diffuse halo around RCW 71 of up to 12 arc. min distance from the core of the nebula.


Data and initial calibration/integration: Alexandr Zaytsev https://www.astrobin.com/users/m57ring/

ASA Ritchey-Chretien RC-1000: D=1m, f/6.8 on alt-azimuthal direct drive fork mount, FLI ProLine 16803 with secondary mirror based motorized focusing and automatic de-rotation (Telescope #1 system of ChileScope observatory, Río Hurtado Valley, Chile).

8x Lum + 6x R + 5x G + 5x B guided 300 sec exposures (2h of combined LRGB integral) collected over a single imaging sessions carried out on Apr 9, 2022 using Chilescope Telescope #1 system.

11x Ha + 9x OIII + 10x SII guided 1200 sec exposures (10h of combined HOS integral) collected over 8 imaging sessions carried out on Aug 4, 5, 10, 11, 13, 14, 17, 18 of 2023 using Chilescope Telescope #1 system.

Thus, total of 12h of combined LRGBHOS integral with Chilescope Telescope #1 system.


Image Processing: Mark Hanson https://www.hansonastronomy.com


Enjoy, Mark and Alex 


[1] http://galaxymap.org/cat/view/rcw/71

[2] https://arxiv.org/abs/astro-ph/0412602

[3] https://arxiv.org/pdf/astro-ph/0412602.pdf

[4] https://arxiv.org/abs/1107.2298

[5] https://arxiv.org/pdf/1107.2298.pdf

[6] https://www.astrobin.com/q16jx0/

[7] https://irsa.ipac.caltech.edu/applications/MSX/MSX/imageDescriptions.htm

[8] https://iopscience.iop.org/article/10.3847/0067-0049/225/1/1

[9] https://iopscience.iop.org/article/10.3847/0067-0049/225/1/1/pdf

[10]  https://archives.esac.esa.int/hsa/aio/jsp/product.jsp?RETRIEVAL_TYPE=POSTCARD&PROTOCOL=HTTP&OBSERVATION.OBSERVATION_OID=8477811

[11] https://archives.esac.esa.int/hsa/aio/jsp/product.jsp?RETRIEVAL_TYPE=POSTCARD&PROTOCOL=HTTP&OBSERVATION.OBSERVATION_OID=8487017