6. Discussion and future plans
The main goal of our long-term project is to study both the LPVs and the photometric variations of a sample of bright southern SPBs. Most of our targets were found as a by-product of the Hipparcos-mission, since their photometric variability was only discovered by analysing their Hipparcos light curves. After two years of monitoring (candidate) B-type pulsators, a handfull of eccentric binaries with a short orbital period and a pulsating component is found.
Recently, Harmanec et al. (1997) started the SEFONO project: a spectroscopic search for forced nonradial oscillations. They started to monitor known binaries with eccentric orbits and orbital periods shorter than some 30 days, which have early-type primaries. In this way, the pulsational character of some of the components is discovered after the detection of LPVs. Our study can be seen as complementary to the SEFONO project, with a completely different starting point.
In the past, many other authors have tried to find evidence for forced oscillations in binaries. Spica ( Vir, SpT B1III-IV) is one of the first discovered double-lined spectroscopic binaries. It consists of two B stars revolving in an orbit with moderately high eccentricity of about 0.15 and with an orbital period of approximately . In two important papers, Smith (1985a, b) reports about high-resolution Reticon observations of the SiIII-triplet at 4552, 4567, 4574 Å. The pattern of moving bumps seen in these spectra can be accounted for by four nonradial pulsation modes. Two of them are related to the orbital motion: (1) a retrograde, tesseral, "quasi-toroidal" mode with a period equal to about 1/12 of the orbital period; (2) an , retrograde, sectoral mode with a period equal to half the orbital period. The latter arises from the tidal distortion of the primary and is the spectroscopic equivalent of the ellipsoidal light variabibity. In our sample, we find three new ellipsoidal variables. For two of them, we see clear LPVs, but no relation with the orbital motion is found (yet).
Mimosa ( Cru, SpT B0.5III) was already a suspect spectroscopic binary for a long time with a suggested orbital period of the order of years. Only recently, Aerts et al. (1998) derived sufficiently accurate orbital parameters. They found an eccentric orbit () with an orbital period of 5.00 years. The system is seen at low inclination and the secondary is a B2V star. In their numerous high-resolution CAT spectra spread over more than 11 years, they only found clear evidence for the presence of the photometric frequency c/d (Cuypers 1983) in two observation-runs which are about one orbital period apart. The variations of the spectra near periastron are dominated by c/d. At all other orbital phases, they find a spurious frequency resulting from a combination of and c/d. These findings suggest that the complicated pattern of pulsation frequencies is affected by tidal forces, but their data is not well enough spread in time to make definite conclusions. In our sample, we find 5 binaries with an eccentric orbit. Especially for HD140873 and HD177863, we find an extremely high eccentricity of respectively and . Therefore, these systems are very promising candidates to search for frequency changes in function of the orbital phase. The time sampling of our current data set is not suited to do this.
Now that the spectra of our binary targets are shifted to the velocity frame relative to the primary component, we are able to start a thorough analysis of the intrinsic variability for both our binary and single targets. It is our final aim to disentangle the frequency spectrum of the SPBs and to identify the modes. If we succeed in doing so, we will have a better view on the selection of the pulsation modes in this class of g-mode pulsators. Moreover, a well-determined frequency spectrum of identified modes is a fruitful starting point for a successful application of seismological techniques.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000