We have obtained and analysed photometric and spectroscopic data for two peculiar emission-line stars AS 78 and MWC 657. Both objects were found to display noticeable photometric variations in the optical region with an amplitude of about 0.3 and 0.6 mag, respectively. High-resolution spectra were obtained for both objects for the first time. We find that they both lack photospheric lines and exhibit variations in the emission line profiles. The IR excesses are also similar in AS 78 and MWC 657 indicating a lack of far-IR radiation. Both objects were identified in the recent mid-IR galactic plane survey of the MSX satellite with fluxes comparable to those of IRAS.
A near-IR excess is detected for AS 78 for the first time. A gradual
decrease of its U-B and B-V and simultaneous increase of the
V-R and V-I colour-indices was observed during more than 6 years of
observations. Variations in the near-IR radiation on the order of
1 mag were found with a time scale of weeks. The radial velocity
measurements in this spectrum suggest that the distance towards AS 78
is 2.9 kpc, based on the galactic rotation curve.
The consideration of different hypotheses about the nature and evolutionary
state of AS 78 enabled us to discard those of a B[e] supergiant, Herbig Ae/Be
star, and post-AGB object. The observed variations of the near-IR
flux are more consistent with the binary hypothesis. If this is the case,
then the primary is responsible for the bulk of the optical radiation and
had
K at the epoch of our first spectroscopic
observations (January 1994);
and/or the stellar wind strength
appears to be increasing with time.
The high-resolution spectrum obtained in 1999 indicates changes in the wind density structure in comparison with the 1994 data. The secondary must be a less luminous star which is probably surrounded by an optically-thick and geometrically-thin gaseous and dusty disk. Although we currently do not have sufficient evidence for the object's binarity, the observed strong emission-line spectrum of AS 78 is rather unusual for such a relatively low-mass star.
Cyclical photometric variations with a period of 86 days were suspected for MWC 657. The optical colour-indices tend to become bluer as the star brightens in the V-band. The K-band brightness tends to increase significantly towards phase 0.5, which corresponds to a weak secondary minimum. The double-peaked Balmer emission lines probably show phase dependent V/R changes. We suggest that all these phenomena might be due to the orbital motion in a close binary system, where the gaseous disk around the invisible star fills its Roche lobe and pours matter onto the visible component. The latter is probably a B-type giant star. Such a system is rather unusual, but not completely unexpected.
The observations obtained so far do not allow us to derive the objects' parameters with high accuracy. Additional observations including optical and IR photometry and high-resolution spectroscopy are required for both stars. Nevertheless, this study has revealed a number of their important properties, and our results allow us to create an observing strategy for both. In particular, frequent near-IR observations seem to be the most important to get more information about the IR emitting regions and possible secondary components.
Acknowledgements
We are grateful to B. Kallemov and T.M. Pogrosheva for their help in the photometric observations, N.V. Borisov for his help in the spectroscopic observations with the TV scanner, N.E. Kurochkin for his work with the Moscow plate archive, and P. Corporon for providing us with his high-resolution spectra of MWC 657. A.M. and K.S.B. acknowledge support from NASA grant NAG5-8054 and thank the IRTF staff for their assistance during the observations. Karen Bjorkman is a Cottrell Scholar of the Research Corporation, and gratefully acknowledges their support. P.G.-L. acknowledges support from grant PB97-1435-C02-02 from the Spanish Dirección General de Enseñanza Superior e Investigación Científica (DGESIC). V.K. acknowledges support from grant 99-02-18339 of the Russian Foundation for Basic Research. R.J.R., D.K.L., and S.M. were supported by the Independent Research and Development program at The Aerospace Corporation. This research has made use of the SIMBAD database operated at CDS, Strasbourg, France.
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