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Astron. Astrophys. 327, 1185-1193 (1997)

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5. A search for young stars

Because of the obvious influence on the globules of the ambient medium, it is possible that the formation of stars has taken place in the globules or in the lower-density molecular cloud material, which once enveloped them. However, our objective prism plates show no H [FORMULA] emission stars in direct association with the globules. This is perhaps not surprising, since the ability of objective prism plates to pick up H [FORMULA] emission decreases with distance, and at 1.8 kpc only very intense H [FORMULA] can be expected to be recognized. Seven H [FORMULA] emission stars were found in the HII region, and they are identified in Fig. 1, and their positions are listed in Table  2.

[TABLE]

Table 2. Emission line stars in IC 2944

The identification of ESO H [FORMULA]  301 on the direct plate was difficult, and is not completely certain. Most, if not all, of the emission line stars are physically associated with the HII region, because in a search of random areas of the same size on the same plates one finds between 0 and 1 emission line stars. The seven stars all show strong H [FORMULA] emission. The majority of these stars are probably young pre-main sequence stars of intermediate mass, presumably similar to Herbig Ae-Be stars. There is no H [FORMULA] emission star associated with or even near the globule complex. It should be noted that the survey for emission line stars around the globules was hampered by the glare of HD 101205, and cannot be considered complete. Slit spectra in the spectral range [FORMULA] 4200 A to 6750 A [FORMULA] were obtained of the H [FORMULA] emission stars, and in Table  2 we list the H [FORMULA] equivalent width. Only one star, ESO H [FORMULA] 302, shows a rich emission line spectrum, with numerous FeII lines in emission in addition to the Balmer lines (see Fig. 8). This star is located just outside a large windswept cloud structure at the western edge of the HII region, and was probably born as a result of the compression of the neutral material by the advance of the ionization-shock front.

[FIGURE] Fig. 8. A spectrum of the rich emission line star ESO H [FORMULA]  302. Besides the Balmer lines, numerous FeII lines are prominent

Three stars are visible within the outline of Thackeray 1. The deep H [FORMULA] CCD images show no reflection nebulosity around the stars, and they are presumably line-of-sight associations. Moneti (1991) searched the globule for embedded infrared sources, but found that all sources within the outline of the globule are consistent with highly reddened background late-type stars.

Thackeray (1955) drew attention to a star which between 1950 and 1954 had faded from about 15.5 mag to about 18 mag (photo-visual magnitudes). An intermediate plate taken in 1951 showed it at about 17 mag. Because of its location at the sharp eastern edge of the larger globule, Thackeray considered it possible that he witnessed the actual obscuration of the star behind the cloud-edge, due to relative motion of star and globule. The star has been identified on the present images and on this higher resolution material it is clear that the star is several arcseconds from the sharp globule edge. The star is seen in Fig. 3 at the same declination and immediately to the left of the sharp northern peak of Thackeray 1. The red filter/plate combination used by Thackeray is not quite the same as used for Fig. 2a, but, for what it is worth, comparison between the variable star and neighbouring stars shows it at about the same relative magnitude as in 1954.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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