Abstract
The cometary globule CG 12 and its associated young cluster NGC 5367 is located at a Galactic latitude of b = 21°, and the origin of this high-latitude star formation event has been unclear. Getman et al. identified a nearby group of B stars and suggested they could be the remnant of a group of massive stars of which one exploded as a supernova. We here use Gaia EDR3 data to measure the accurate distance of NGC 5367 and the B-stars. We find a mean distance of 670 pc for NGC 5367 and 675 pc for the B-stars, strongly supporting the suggestion of Getman et al.
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1. Introduction
The magnificent cometary globule CG 12 was discovered by Hawarden & Brand (1976) at an unusually high latitude of b = 21°. Cometary globules generally represent secondary star formation regions facing toward groups of massive young stars (Reipurth 1983). Williams et al. (1977) showed that the CG 12 globule harbors a group of young stars known as NGC 5367, leading to the question what could have triggered this isolated star formation and the globule's prominent, more than 1 pc long, tail. The cometary globule is facing away from the Galactic plane, so it is not a case of a gas clump falling into the plane. Subsequently, CG 12 has been studied at mm-wavelengths by White (1993), Haikala et al. (2006), and Haikala & Olberg (2007). Additional embedded young sources were found by Santos et al. (1998) and Haikala & Reipurth (2010), indicating ongoing star formation. Most recently, Getman et al. (2008) carried out an X-ray study of CG 12 and identified ∼50 more young stars, indicating a much higher star formation efficiency than normally found in cometary globules. These young stars have a large spread in ages up to ∼20 Myr. The distance to CG 12 is uncertain, with suggested values ranging from ∼300 pc (van Till et al. 1975) to 660 ± 100 pc (Marraco & Forte 1978). The most recent determination suggests ∼550 pc (Maheswar et al. 2004).
2. Results and Analysis
It has long been recognized that the cometary appearance and star formation activity of CG 12 could be the result of a nearby supernova explosion, and it has been speculated that it might be associated with an H I shell centered at l = 315°, b = 30° (Williams et al. 1977). However, Getman et al. (2008) identified a much closer group of six B-stars, to the northwest and ahead of the globule. All of them are late B-stars with the exception of HD 120958, which is a B3 Vne star. It is also the star closest to CG 12, and located on a line precisely aligned with the tail direction of the cometary globule. Getman et al. (2008) suggest that this group of B-stars in the past might have included one or more massive stars, which could have exploded as supernovae during the past ∼20 Myr, thus forming the cometary appearance of CG 12 and triggering the formation of its young stars. This appealing scenario rests, however, on the assumption that the HD 120958 group is located at the same distance as the cometary globule. We here present a precise determination of the distance to CG 12 and to the HD 120958 group using the Gaia EDR3 catalog.
Table 1 contains a list of 6 stars known to be associated with the globule, and 2 X-ray sources classified as highly probable members by Getman et al. (2008). They were selected from the large number of young optical and near-infrared sources in the region because they have parallax error-estimates of 0.03 mas or less. The mean distance of these 8 stars is 670 pc with a standard deviation of 10 pc and a standard error of 4 pc. Similarly we have used the Gaia EDR3 parallaxes for the full set of 6 B-stars to determine the distance to this group. The parallax errors for these stars are higher, ranging from 0.04 to 0.09 mas. The mean distance of the B-stars is 675 pc with a standard deviation of 33 pc and a standard-error of 13 pc. Thus, the mean distances along the line of sight to CG 12 and the group of B-stars differ by only around 5 pc.
Table 1. Parallax Measurements of NGC 5367 and Associated B-stars
| Identifier | Alternative ID | Gaia EDR3 | Parallax (mas) |
|---|---|---|---|
| herschel 4636S a | CD-39 8581 | 6114205804964036096 | 1.482 ± 0.020 |
| herschel 4636N a | CD-39 8581 | 6114205804964036352 | 1.459 ± 0.016 |
| CXOU J135711.3−400234 | [GFL2008] I-9 | 6113841832256080256 | 1.464 ± 0.014 |
| CXOU J135712.0−400050 | [GFL2008] I-10 | 6113842038414511872 | 1.474 ± 0.033 |
| NGC 5367 2 | CD-39 8583 | 6114204396214746240 | 1.510 ± 0.030 |
| NGC 5367 5 | 6114205942402994688 | 1.578 ± 0.013 | |
| NGC 5367 6 b | 6114206011122475264 | 1.510 ± 0.019 | |
| NGC 5367 8 | 6114217933951693696 | 1.459 ± 0.017 | |
| HD 120958 | 6114176599182763648 | 1.376 ± 0.066 | |
| HD 120124 | 6115001610858483968 | 1.650 ± 0.044 | |
| HD 119627 | 6115111184064386688 | 1.508 ± 0.035 | |
| HD 119484 | 6115077717678805888 | 1.611 ± 0.086 | |
| HD 119338 | 6115173821867443712 | 1.400 ± 0.047 | |
| HD 119277 | 6115171824705184640 | 1.389 ± 0.061 | |
Notes.
a h4636 appears to have the distinction of being the first star, known today as a YSO, that was resolved as a binary (Herschel 1847). b NGC 5367 6 is a binary and the value refers to the northwest component. The parallax of the southeast component exceeds the limit of 0.03 mas.We can also ask what is the specific physical separation of CG 12 and the nearest and most luminous of the B-stars, HD 120958. If the star is at the same distance along the line of sight as CG 12, their physical separation is only 15 pc. Unfortunately, the Gaia parallax for this star is by far the most uncertain of the stars discussed here, but if we accept its observed parallax, their physical separation is 54 ± 37 pc.
The Gaia EDR3 parallaxes thus lend strong support to the suggestion by Getman et al. (2008) that CG 12 is associated with and impacted by present and past members of this group of massive stars.
At a distance of 670 pc, CG 12 is located about 240 pc above the Galactic plane. Although the question of how star formation started in CG 12 now appears to have a clearer answer, another question is how the HD 120958 group of massive stars originated so high above the Galactic plane.