Page 29 - spectroscopic-atlas-5_0-english_Neat

Spectroscopic Atlas for Amateur Astronomers 29

Most of the absorption lines appear here similar to those of Alnitak and Mintaka. Hβ is strik-
ing, because the emission line, generated in the surrounding nebula, grows out of a broad,
photospheric absorption dip of the star. The two [O III] emissions must be generated by the
surrounding nebula, because the hot and dense stellar atmospheres can impossibly gener-
ate "forbidden" lines. The Hα emission is produced by the recombination of the encircling H
II region, totally veiling here the stellar line (sect. 28).

It is strongly recommended to record Θ1Ori C with autoguiding to ensure the tracking on
the correct trapezium star. The orientation of the slit axis should be optimised accordingly.
The sketch above should facilitate the orientation. The exposure time for the profile in Table
3 is 340 seconds.

68 Cygni (~2300 ly) is surrounded by the weakly developed H II region Sharpless 119. It is
the brightest of total 6 ionising stars. Most of the sources classify this star with 30 solar
masses and a surface temperature of about 35'000K as O7 III n (f) [505]. “Of” stars are
considered as a transitional phase on the way to Wolf Rayet stars and show eg He and N
lines in emission - a clear indication that material is repelled. As classification lines for the
Of-type the blends N III λλλ 4634–40–42 and He II λ 4686 are used [2]. Since only N III ap-
pears here in emission, 68 Cygni is considered as a "mild" Of star, and the suffix "f" is
therefore set in parentheses (f) [2]. In the intensive Of -phase both features, N III and He II,
show up in emission. Compared to the O9-types the C IV lines (λλ 5801/5812) are quite
intensive here. This indicates that 68 Cygni, as well as Θ1 Ori C, are stars of the rather early
O-class. C IV requires with 47.9 eV almost twice the ionisation energy as He II.

According to [505] 68 Cygni passed about 1959-70 a phase with Hα in emission. Because
of the high proper motion, it is regarded as so called "runaway star" and originates probably
from the OB2 region of Cepheus. As a possible scenario, the acceleration by the supernova
explosion of a companion star is discussed. Perhaps the remaining black hole with ~3
M still orbits around the star very closely. Observed fluctuations of equivalent widths at
certain spectral lines indicate a possible orbital period of about 5 days. The prior informa-
tion is mainly based on [352].