Shedding light on Bernes 157

Shedding light on Bernes 157

by Rodolfo Ferraiuolo & Enzo De Bernardini

(versión en español)

An intricate area of our galaxy called CrA Molecular Complex stretches, for about 8° x 3°, over the constellations of Corona Australis and Sagittarius. It visually starts from the furthest and the most darkened globular cluster NGC 6723 in Sgr and continues, very faint, by CrA. It then develops to the east and southeast, returning inside the boundaries of Sagittarius, and passing behind the stars of gamma (γ) CrA (a yellow-white binary of magnitude 4.2) and alfa (α) CrA (Alfecca Meridiana, a white star of magnitude 4.1).

The complex, of fifty solar masses of molecular gas, stretches through several light years, being a good area for star formation, where various Herbig-Haro objects and H2O molecules have been detected by means of infrared observations.

Curiously enough, the densest and most interesting area of the Complex, with about 1.5 square degrees, is wrongly called Bernes 157 in most books, softwares and astronomical charts. This is one of the reasons why we have decided to explore Be 157 in deep: to reveal its real identity by studying some bright and some dark associated nebulosity.

In order to place ourselves in the said area, we might start by locating the globular cluster NGC 6723 with our telescope using low magnification and/or a wide-field eyepiece. We will follow an observational route towards the southeast to get in Corona Australis, happening upon a captivating image from the beginning. We will notice a drastic drop in the previously populated star field from about 12’ from the cluster, signaling that the dark nebula, whose absorption might exceed eight magnitudes, starts gaining density from a line going northeast-southwest.

As we were saying, this large, dark and irregular area is wrongly called Be 157. It was discovered on July 30, 1826 by James Dunlop (1793-1848), using a 9” Speculum reflecting telescope (nowadays similar to a 6” reflector), from Parramatta, New South Wales, Australia. The great Scottish Astronomer recorded the area under number 559 in his catalogue , and described it as "A singular dark space in the heavens of an irregular figure, about 1½° long, and 1½° broad; no stars except exceedingly minute stars in the greatest portion of this space”. It is situated in (19h 02m 54s; -37º 08'_J2000.0), between the yellow stars gamma (γ) and epsilon (ε) CrA.

By mid 1970’s, the Swedish astronomer, Claes Bernes, observed different nebulosities in the area and originally termed Be 157 as a small reflection nebula, with a diameter of 1’ shrouded in this complex area of absorption, together with some other bright nebulae. This small irregular object, also catalogued as Magakian 782, is in fact the small nebulosity surrounding the variable star V 709 CrA (GSC 07421-01890), of magnitude 11.3-11.7, situated in (19h 01m 34.8s; -37º 00' 56.6"_J2000.0), between NGC 6729 and IC 4812. It was wrongly located, with an error of almost 2 arcminutes in its original publication “A Catalogue of Bright Nebulosities in Opaque Dust Clouds”, (C. Bernes, 1977).

Later on, because of reasons which are not clearly stated, the term for this small nebular object (Be 157), was used to refer to the whole area.

The other bright nebulosities studied by C. Bernes and which belong to this area are: Be 158 (HH 100), a faint Herbig-Haro object, situated in (19h 01m 49s; -36º 58' 16"); Be 159 (NGC 6729) and Be 160 (NGC 6726/27).

In order to avoid confusion, we will refer to this large area of absorption as DN Be 157 (DN for Dark Nebula) from now onwards.

DN Be 157 is situated about 450 light years from us, with the most conspicuous area taking up a surface of 55’ x 18’ in the sky. The densest part of gas and dust has a real diameter of about 8 light years and it is situated towards the southeast of the reflection nebulosities NGC 6726/27 and NGC 6729, stretching linearly towards that direction. The starless space is easily noticeable there, observing total darkness with some interesting and beautiful opaque areas of brownish hue. About 12’ to the east-southeast of IC 4812 we will find the variable star V 702 CrA, of 10th magnitude. Towards its east, a branch appears as a dark and dense extension of DN Be 157, going from northeast to southwest by about 15’ and with an irregular width of about 6’. We will also notice some other dark branches, such as SL 39, 12’ to the southeast of the former.

Image by Leonardo Julio (Takahashi Epsilon 180, Canon 5D Mark II Hutech, 46 x 4 min @ ISO-1600,
from Intendente Alvear, La Pampa, Argentina)

Apart from that, on the west side of the nebulae NGC 6726/27 and IC 4812, a quite dense part of the dark nebula runs parallel to them and between the said nebulae and the globular cluster NGC 6723. Catalogued as SL 41 by A. Sandqvist and K. P. Lindroos, it is about 45’ long and 20’ wide, between the star SAO 210834 (HD 176423) of magnitude 9.1, situated 13’ to the north of NGC 6727, and epsilon (ε) CrA, situated 28’ virtually to the west of IC 4812. The latter star is also a beautiful binary of two yellowish components, discovered by John Herschel (1792-1871) in the year 1834.

For a better visual study of DN Be 157, it is advisable to move away from the luminous pollution of our cities and use a reflecting telescope with a minimum aperture of 4”.

Through the telescope and among this darkness, two nebulosities – NGC 6726 and NGC 6727 – appear about 28 arcminutes to the southeast of the globular cluster NGC 6723. They were discovered with a 6.2” Plössl refractor in the year 1861 by the German astronomer Johann Friedrich Julius Schmidt (1825-1884) when studying NGC 6723. Actually, they were the same reflection nebula occupying an area of 2.5’ x 1.5’. It shines thanks to two hot stars which illuminate the surrounding gas and interstellar dust, dividing the nebula into two main smaller parts. Both stars are spaced by 58”, directed north-northeast to south-southwest, PA 198°. They are very young, of the Herbig Ae/Be type, shrouded in gas and dust, probably having protoplanetary disks. These stars are undergoing the phase of gravitational contraction, currently in the pre-main sequence (PMS) of the H-R diagram and getting closer to the Main Sequence. This type of warm and young celestial bodies which are less than 10 million years old and type A or B is termed HAeBe. It was named after the American astronomer George Howard Herbig, who differentiated them from other stars of a similar evolutionary phase in 1960.

The faintest star, to the northeast, illuminating the nebula NGC 6727, is TY CrA (SAO 210829), an interesting eclipsing variable, which is a closed binary, with a magnitude ranging from 9.39 to 9.81. It has a third confirmed component, and also a possible fourth component, orbiting the main binary. The main component – type B9Vea, dominates the luminosity in the system. The mysterious dynamic of each component’s constrained and unstable orbits is currently researched with a great interest. The main component of TY CrA has about 3 solar masses, the secondary one, 1.6 solar masses and the third one, 2.4. The orbit of the latter has an inclination of 70° regarding the orbital plane of components 1 and 2. The third component might be a red dwarf star. We should also underline that several X-ray emissions have been detected in the binary system.

The brightest star, to the southwest and illuminating NGC 6726, is SAO 210828 (HD 176386), of magnitude 7.2 and spectral type B9IVe, about 448 light years from us. This star is also found in the Pre-Main Sequence, the same as TY CrA, and it might be a binary or a multiple system.

As we said before, NGC 6726/27 is only one nebulosity, made up of two rounded areas which come into contact with and are illuminated by the stars TY CrA and SAO 210820. This couple of stars might be spotted as two separate bodies by using 15 x 70 binoculars. The nebula is easily seen with 3.2” refractors and with reflectors of more than 5”. This nebulosity, like IC 4812, appears in photographs with a bluish hue and similar to the nebulosities of the Pleiades.

By observing this target with a 10” telescope, the nebulosities will be of a moderate brightness and grayish hue, making an ‘8-shape’, with blur borders which will be more definite by using averted vision. To the south and southwest borders of NGC 6726 there is a sudden drop in brightness, and the nebulosity looks as a small bite because of an intense darkening caused by interstellar dust. An UHC filter will highlight the contrast.

Only 5’ to the southeast of NGC 6726/27, we will find our next interesting target, the nebula NGC 6729.

It was also discovered by Johann F. J. Schmidt in 1861, from the Athens Observatory, Greece. It was then discovered independently by the German astronomer Albert Marth (1828-1897) in 1864 from the observatory in the Island of Malta. This reflecting variable nebulosity is fascinating and tangled, with minimal emission areas, illuminated by the young irregular variable R CrA.

This dense nebula, also called R CrA Nebula, has an apparent dimension of 1.5’ x 0.8’and a comet-like appearance with a northwest-southeast orientation. R CrA is situated in the northwest end of the nebula, as if it were the core of the comet. Its variation ranges between 9.7 and 13.5, with some sudden peaks of brightness, making the brightness of the nebula vary. It might be a protoplanetary disk with a diameter of no more than 450 Astronomical Units. The fast movement of the nearby clouds of dust and gas of the enormous nebula (which might start at only one Astronomical Unit from the star) might be the ones which darken the star, as a play of light and shadow, causing the very irregular variation of R CrA brightness, similar to what happens in the Hubble’s Variable Nebula (NGC 2261). R CrA is another very young protostar of the type HAeBe, twice more massive and forty times more luminous than our Sun. Two fast bipolar jets of plasma have been detected emerging from it.

Apart from R CrA, there is another variable star called T CrA, which is involved with the nebula and situated 1.2’ to the southeast of R CrA. It is in and close to the border of NGC 6729, in ‘the tail of the comet’. The brightness of this star ranges between 11.7 and 13.5.

We will observe NGC 6729 with some difficulty with 2.8” refractors and with reflectors of more than 4” from a dark and clear sky. By using an 8” reflector at 80x we will see it faint and weird, small and elongated, with comet-like appearance. With an 11” SC it will still be small, of about 1’ x 0.5’, but it will be interesting at low and high magnification, and the nebular connection between both stars will be barely noticeable.

To finish with our study of NGC 6729, we will now go back to NGC 6726/27, to move about 12’ to the southwest until we find a reflection nebula. This is IC 4812, with an apparent diameter of 10’, being part of the same gas and dust as NGC 6726/27.

It was discovered in the photographic plates of the American astronomer DeLisle Stewart (1870-1941), with a 24” Bruce astrograph telescope, from the ancient Arequipa Station of the Harvard College Observatory, Perú. The brightest area of this nebula stands out visually, and it is associated with and situated around the beautiful and bright double star, of white-bluish color, BSO 14 (Brisbane Observatory Catalogue), with magnitudes 6.4 and 6.7. This pair of stars is visible as a single star with 7 x 50 binoculars, but it is solved with small apertures. It is directed east-southeast to west-northwest, PA 282°, rough separation of 13”. Its brightest component, SAO 210816 (HD 176270), situated to the east, is of spectral type B8IV-V, and its companion, SAO 210815 (HD 176269), is type B9V. By observing this stellar pair with apertures bigger than 6”, it will appear bluish and of similar brightness, being IC 4812 easily noticeable around it from a dark place as a faint and small irregular nebulosity, with a diameter of 1.5’ and surface brightness of 12.8. With apertures bigger than 10”, the nebula will gain very little in size, getting at almost 2’, where an UHC filter will be useful again. Around IC 4812, especially to the south, southeast and southwest, we will see some faint nearby stars since the absorption of the dark nebula we have come to call DN Be 157 decreases.

Finishing our visual journey through this complex and charming area, we should add that nebula Be 157, the original one, is not easily seen and its shape is ideal for astrophotographers.

Right Ascension and Declination are for equinox 2000.0.

Referencias:

• A catalogue of bright nebulosities in opaque dust clouds. Bernes, C.
  AA (Stockholms Observatorium, Saltsjobaden, Sweden).
  Astron. Astrophy. Suppl. Series, vol. 29, July 1977, p. 65-70. 07/1977.
• Catalogue of dark nebulae and globules for galactic longitudes 240 to 360 degrees. J. V. Feitzinger and J. A. Stüwe.
  Astronomisches Institut der Ruhr-Universität Bochum. Germany.
  Astron. Astrophy. Suppl. Ser. 58, 365-386 (1984).
• Interstellar Formaldehyde in Southern Dark Dust Clouds. Aa. Sandqvist and K. P. Lindroos.
  Stockholm Observatory, Sweden.
  Astron. Astrophy., 53, 179-189 (1976).