4. Discussions of individual stars
4.1. Emission line stars
Three stars show emission spectra in our sample, i.e. # 33, 125 and 151; of these only #125 (HD229221, BD+38 4062) has been previously classified as B0Ipe (Morgan et al. 1953). Roman (1951) classified #125 as B0IIe. Morgan et al. (1953) classified it B0Iabpe in the study for a group of blue giants. Based on its spectrum, we classify it B0IIIe. Its mass could be around 8-10 ; this would make it an intermediate mass star. Because it is a member of NGC 6913, it could be a Herbig Be star candidate. As the value of E(B-V) of #125 is 1.17, a dense circumstellar disk is thus inferred. In addition, Wang & Hu (1996) found that the spectrum of #125 underwent a peculiar variation, from a single emission line to one composed of three components, i.e. emission + absorption + filled emission. They concluded that this could be explained by a model of a central star + asymmetrical circumstellar disk or clump + outer layer of dust. The star could have planets forming around it.
#33 shows H in medium-strong emission and H and H in absorption. Its spectral type is A0Ve. The value of E(B-V) equals 0.71. It seems to be a classical AeBe star.
#151 shows a very large value of E(B-V) (1.47). It is classified as B5e. The Intensity of the H emission line is very high. It could have a thick envelope. The star is also inferred to be a pre-main-sequence star with intermediate mass (Herbig Ae/Be star).
4.2. The stars with peculiar colour excess
There are a few stars with peculiar colour excess, i.e. #28, 51, 179, 187 and 32, some of which show negative values of E(B-V). In addition, some stars show relatively low positive colour excess values.
#28 is classified as a K2III star. It has very strange observed colours (B-V) and (U-B). The value of the former is the highest in all the cluster stars, attaining to 1.85, and the value of the latter is relatively low (0.33). The intrinsic colours of (B-V) and (U-B) are 1.16 and 1.17 separately. Thus a value of E(U-B)=-0.84 is derived. If the spectral type of #28 is earlier than G2 to make the value of E(U-B) greater than zero, then the value of E(B-V) would be greater than 2.5! In this case, #28 would be a variable star with large light variations and with a extremly thick envelope (the UBV photometry was carried out 11 years ago). Otherwise, if it is not a variable star, and the photoelectric photometry in UB bands is reliable, then it must be a extremely " blue-enhancing " star. It should be paid close attention to and should be observed further.
#32 has E(B-V) just below zero, and #51 and 187 have a relatively large negative E(U-B) colour excess. Both E(B-V) and E(U-B) of #179 appear large negative values. #131, 135 and 144 have low positive values of colour excess (less than 0.2). What causes the very low or even negative values for the colour excess?
Clearly, all of these stars except one (#187) have a spectral type later than F0 (#187 is a late A type star). There are three possible explanations for this phenomenon. (1) Six out of the seven non-early-type stars have relatively low V photometric values. It is difficult for a 1-meter class telescope to measure the faint blue light of the late-type stars by using photoelectric methods; this could cause a large uncertainty of the photometry in the blue bands. (2) Young stars sometimes show spectral blue veiling and ultraviolet excess. These effects make the (B-V) colour too blue compared with its spectral type and a negative value of colour excess E(B-V) for the star will be observed if the blueing effect of the blue continuum is greater than the reddening effect of interstellar absorption. A few stars in the Orion nebula (NGC 1976) have been observed to have such values (Walker 1983). (3) They are possible variables with large light variations.
Here two stars are worthy of special mention, i.e. #135 and 179. #135 has a membership probability of 76%. It lies in the central area of the cluster. It is a very luminous F0III star. Its V magnitude is 8.57, which makes it one of the brightest stars in the NGC 6913 field. Nearly all of the stars in the field with large V magnitude are early-type stars (OB stars). The value of E(B-V) is very small (0.15), much smaller than the other stars with large V magnitude. #135 might not be a member of the cluster, probably a foreground star.
#179 has a high membership probability in this area (88%). Its V magnitude is 13.84 mag. Its position is not far away from the central region. Although it is a K-type star, the value of its reddening is negative! It must be a variable star.
4.3. The possible variable stars with large amplitude
Comparing with Joshi's photometry, we found a few stars to have more than 0.4 magnitude difference in Vilnius photometry on V band, namely # 40, 119, 120, 123, 136, 141 and 155, which have values of Vilnius V magnitude 13.09, 12.76, 13.59, 13.53, 12.20 13.60 and 13.16 respectively (ref. Vilnius photometry database, Mermilliod 1995). Most of them are A type stars. Two of them are B type stars. Especially, there is a remarkable variation for another B star # 155, which changes from 8.91 mag. in Joshi's photometry to 13.16 mag. in Vilnius' photometry! If no identification mistake was made, it should be a very interesting variable star.
© European Southern Observatory (ESO) 2000
Online publication: March 28, 2000