An arc-shaped emission nebula (referred to as the arc in the rest of the text) in the constellation Gemini with dominant OIII lines and an apparent length of about 14° is presented on this page. The object has already been recorded in far UV surveys. What is new, however, is that this object can be seen in visible light and that it is a strong OIII emitter. This topic is discussed in the section “Classification and discussion of the superstructure”.
Click on the image for a full scale version.
This image is a false color composite where [OIII] (505.7 nm) is mapped to red, Hα is mapped to green, and [SII] (671.7 nm and 673.0 nm) is mapped to blue. All three color channels contain some continuum light, which appears magenta in the image due to the choice of the color factors.
Stars are partially subtracted (with different factors for each color so that the average star color is white).
The southern (lower) part of the large arc-shaped object that extends across the entire image is only visible in [OIII] light (red). A smaller part, about 3° in diameter, is only bright in Hα and [SII] light (cyan).
Some fainter filaments (red and bluish) on the left side are visible in all three wavelengths and are probably related to the large arc.
The bright object at the lower right is SH2-274. Also note that there are a lot of weakly ionized background nebulae (HII and [SII], green to cyan).
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this link for an annotated presentation of the mentioned objects.
![]() [OIII] | ![]() Hα |
![]() [SII] | ![]() Regions for luminance measurements |
The following table contains the results of intensity measurements for the regions marked in the upper image. The average intensity in Rayleighs is given in each case. The region labeled “bg” was used as a background reference.
Region | Hα | [OIII] | [SII] |
---|---|---|---|
K1a | 0.02 | 2.04 | 0.06 |
K1a' | 1.68 | 1.53 | 1.14 |
K1b | 2.20 | 0.63 | 2.27 |
K1c | 0.24 | 0.72 | 1.54 |
For the determination of the intensity, some parameters had to be estimated. The transmission ratio of the optics and filters was assumed to be 75% for [OIII] and Hα and 70% for the [SII] doublet. The atmospheric extinction under best conditions was assumed to be 20%. The errors resulting from these estimations only distort the absolute measured values by a factor, but have no influence on the intensity ratios between the wavelengths. More relevant are the errors caused by the background calibration, as they distort the intensity ratios between the individual emission lines. The accuracy of the background reference (the two regions marked “bg” in the image above) is limited by the fact that the entire region is full of faint background nebulae.
The image is centered on the pulsar PSR B0656+14, the suspected origin of the Monogem ring. The arc could belong to that Supernova remnant, as discussed below.
The arc could be a part of the Monogem ring, a supernova remnant (SNR) probably with the same progenitor as the Pulsar PSR B0656+14 (Thorsett et al.). In X-rays, the Monogem SNR appears as a primary ring with a diameter of 18.4° and an eastern blowout, resulting in a size of about 25°. A possible explanation for the distorted morphology is a higher density of the interstellar medium (ISM) near the galactic plane resulting in a slower expansion (Knies et al., 2018). The primary ring corresponds approximately with [OIII] filaments near the galactic plane. The arc lies near the outer boundary of the blowout. If the arc is completed to a ring, its diameter would be about 50°, but its center lies in a similar direction as PSR B0656+14. Thus, if the arc is a part of the Monogem ring, its 3D morphology could be described as egg-shaped, where the narrow end near the galactic plane is formed by the primary ring and the wider end is determined by the arc.
The objects mentioned above can be found in a JavaScript presentation.
The research papers I found use data that cannot resolve the exact orientation of the arc, and I am not aware of any articles that used the high resolution GALEX data of the arc. A full-width view with [OIII] and [SII] data covering the entire ring constructed from the arc could provide further insights. However, complete coverage of the region is not expected before 2025.
A discussion about the superstructure of the arc can be found at Cloudynights forum.
Image data are:
View #1 | View #2 | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Center position: | RA: 7:44h, DEC: 18:30° | RA: 7:59:48h, DEC: 14:14:19° | ||||||||||||
FOV: | 8.75°×14.31° (RA×DEC, through center) | 31°×31° (RA×DEC, through center) | ||||||||||||
Orientation: | North is up | North is up | ||||||||||||
Scale: | 10 arcsec/pixel (in center at full resolution) | 20 arcsec/pixel (in center at full resolution) | ||||||||||||
Projection type: | Stereographic | Stereographic | ||||||||||||
Exposure times (Sum of exposure times of all frames used to calculate the image): |
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The image processing steps were:
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