The young star cluster Berkeley 14A and a molecular cloud to its north are some of the few objects in the Auriga Gap. The nearby Taurus dark cloud complex, part of the Gould Belt, looms in the south.
Source: Galactic Plane Explorer microwave image
There is a gap of almost 12 degrees with little emission at most frequencies in the western outer galaxy from about 168.4° to 156.6°. The SIMBAD astronomical database lists only 9 ionising stars in this region - and of the three O-class stars, one, KR Per, is misidentified - it is actually an F5V star [1]. Most of this gap is in the direction of the constellation Auriga, and so I've called it the Auriga gap.
There are plenty of ordinary stars in this region, but very few of the typical spiral arm tracers such as giant molecular clouds, ionising stars or HII regions. The Auriga gap appears to mark the end of the Perseus arm as a coherent structure. There are still molecular clouds and ionising stars at the distance of the Perseus arm further east, but they appear randomly and only occasionally, lacking the coherent structure of the Cassiopeia arc to the west. Closer to our solar system, within the Orion spur, the Auriga gap also marks an empty region between the Camelopardalis dark clouds to the west and the Gemini molecular clouds to the east.
The Auriga gap begins after the complex of five HII regions Sh 2-225, Sh 2-226, Sh 2-227, Sh 2-228 and BFS45, all of which have roughly similar distance estimates (3500 to 4200 parsecs) that place them behind the western edge of the Aur OB1 association.
At a distance of about 3500 parsecs, Sh 2-228 contains the massive infrared star cluster [IBP2002] CC01. (This is misidentified in SIMBAD as a possible globular cluster.) The 1800 solar mass cluster is 1 to 2 million years old, and the surrounding field stars about 5-6 million years old. [2]
Very little has been published on Sh 2-226 beyond a distance estimate of 4200 parsecs [3] and a comment that it appears much larger in radio emission than hydrogen-alpha. [4]
The small (less than 1 parsec in diameter) BFS 45 is associated with the newly discovered cluster Teutsch 132 and the B-class emission star HBHA 3715-24. [5]
Further north is the mysterious Sh 2-227 (LBN 781), which Avedisova suggests is ionised by the O9V star LS V +38 12. [6] There are no other significant references to this nebula in the scientific literature.
Sh 2-225 contains the young stellar object candidate IRAS 05235+4033. [7]
There are a few interesting objects even in the Auriga gap. The most obvious at radio frequencies is the supernova remnant HB9 (SNR 160.9+2.6). According to a recent paper [8], HB9 is located at a distance of 800 parsecs and is about 4-7000 years old. It is associated with the pulsar PSR B0458+46.
Two other supernova remnants in the Auriga gap are Sh 2-233 (SNR 166.2+02.5) and Sh 2-234 (SNR G166.1 +4.4).
To the southeast of HB9 at about 1100 parsecs is the 9 million year old ionising star cluster Berkeley 14A, which contains three hot B-class stars: HD 31617 (B2IV), DM +43 1161 (B1IV), and HD 31894 (B2IV-V). South of HB9 but much further away at 6100 parsecs [3] is the HII region BFS 44. The bright radio source below BFS 44 is the galaxy 4C 44.12 = 3C 129. [9].
There are many other objects in the direction of the Auriga Gap not associated with the Milky Way's spiral arms, including the 1.28 billion year cluster Berkeley 16 = NGC 1798 at (160.78°, +04.85°) [10], the multiple giant star Capella (G5IIIe+) at (162.5885°, +04.5664°) [11], and visible towards the bottom of the field at microwave (CO) frequencies, the nearby and very dusty Taurus dark cloud complex, which lies at a distance of about 140 parsecs and extends well below the galactic plane [12]. The Taurus dark cloud complex will be described in detail in a future commentary, Guide to the Gould Belt.
Nebulae in this sector
Notes
1. ^ Chen, K.-Y., Williamon, R. M., Liu, Q., Yang, Y., & Lu, L. 1985, Astronomical Journal, The eclipsing binary KR Persei
2. ^ Borissova, J., Pessev, P., Ivanov, V. D., Saviane, I., Kurtev, R., & Ivanov, G. R. 2003, Astronomy and Astrophysics, Discovery of new Milky Way star cluster candidates in the 2MASS Point Source Catalog. II. Physical properties of the star cluster CC 01.
3. ^ Russeil, D. 2003, Astronomy and Astrophysics, Star-forming complexes and the spiral structure of our Galaxy
4. ^ Dickel, John R. 1968, Astronomical Journal, Radio Emission from the Nebula Sharpless 261 and the Surrounding Galactic Region
5. ^ Kronberger, M., Teutsch, P., Alessi, B., Steine, M., Ferrero, L., Graczewski, K., Juchert, M., Patchick, D., Riddle, D., Saloranta, J., Schoenball, M., & Watson, C. 2006, Astronomy and Astrophysics, New galactic open cluster candidates from DSS and 2MASS imagery
6. ^ Avedisova, V. S. & Kondratenko, G. I. 1984, Nauchnye Informatsii, Exciting stars and the distances of the diffuse nebulae
7. ^ Magnier, E. A., Volp, A. W., Laan, K., van den Ancker, M. E., & Waters, L. B. F. M. 1999, Astronomy and Astrophysics, Transitional YSOs: candidates from flat-spectrum IRAS sources
8. ^ Leahy, D. A. & Tian, W. W. 2007, Astronomy and Astrophysics, Radio spectrum and distance of the SNR HB9
9. ^ Krawczynski, H., Harris, D. E., Grossman, R., Lane, W., Kassim, N., & Willis, A. G. 2003, Monthly Notices of the Royal Astronomical Society, Thermal and non-thermal plasmas in the galaxy cluster 3C 129
10. ^ Salaris, M., Weiss, A., & Percival, S. M. 2004, Astronomy and Astrophysics, The age of the oldest Open Clusters
11. ^ Young, P. R. & Dupree, A. K. 2002, Astrophysical Journal, Capella: Separating the Giants
12. ^ Dame, T. M., Ungerechts, H., Cohen, R. S., de Geus, E. J., Grenier, I. A., May, J., Murphy, D. C., Nyman, L.-A., & Thaddeus, P. 1987, Astrophysical Journal, A composite CO survey of the entire Milky Way