HD 152248: Evidence for interaction

7 Evolutionary status

7.1 Spectral types and luminosity classes

Following the criterion of Conti (1973), we use the logarithm of the EW ratio of He I$\lambda$4471 and He II$\lambda$4542 to determine the spectral types of both components of the system. We get for the mean ratio:

$$\begin{eqnarray*}&&\log W'_{\rm prim}(\lambda4471/\lambda4542) = 0.013\ [-0.035,... ...g W'_{\rm sec}(\lambda4471/\lambda4542) = -0.044\ [-0.101,0.006].\end{eqnarray*}$$

This gives a spectral type of O7.5 and O7 for the primary and secondary stars respectively. The values between brackets correspond to a 1$\sigma$ dispersion on the mean value of the EW ratios. The agreement with the previous determination by PGB using IUE data (O$7+{\rm O}$7) is quite good.

Walborn (1972) quoted a supergiant luminosity class for the composite spectrum of HD152248 though he noted that his classification could be vitiated because of the peculiar He II$\lambda$4686 profile. PGB also adopted a supergiant luminosity class for both components of the system. However, the presence of He II$\lambda$4686 and H$\alpha$ in absorption in the spectrum of both stars argues against such a classification. In fact, PGB's assertion was based on the following UV criteria: a very strong emission in the Si IV$\lambda$$\lambda$1394-1403 lines in all the IUE composite spectra and a blue asymmetry and red emission in the N IV$\lambda$1718 line in both primary and secondary spectra. However, as these authors mentioned in Penny et al. (1996), the Si IV$\lambda$$\lambda$1394-1403 doublet is a wind feature that might not be associated with the photosphere. Furthermore, the dependence of the N IV$\lambda$1718 line on wind strength might lead to a more luminous classification in the case of a star with an unusually strong wind. We believe that previous confusion about the luminosity class of this system arises mainly from the fact that the lines adopted as luminosity classification criteria are probably affected by a wind interaction process occurring within the HD152248 system (see below).

Based on the presence of He II$\lambda$4686 and H$\alpha$ in absorption in the spectrum of both components, we follow Mathys (1988) recommendation and, according to a rough determination of the EW of the He II$\lambda$4686 absorption components (from Table 7), we adopt a giant luminosity class for both stars of the HD152248 system. Morrell et al. (1991) further showed that the unidentified $\lambda$$\lambda$4486, 4504 lines display a clear luminosity dependence. An estimate of the intensity of the $\lambda$4486 line in the spectrum of HD152248 also suggests that the stars of HD152248 are giants.

Finally both stars present N III$\lambda$$\lambda$4634-4641 in emission so that, associated with weak He II$\lambda$4686 absorption, an (f) tag should be added to the spectral classification. The new classification for this system is thus: O7.5III(f) + O7III(f).

Figure 10: Hertzsprung-Russell diagram of the HD152248 system. The open and filled symbols stand for the primary and secondary respectively. Evolutionary tracks are from Schaller et al. (1992) for Z=0.020 and adopting `standard' mass loss rates

7.2 Location in the H-R diagram

Adopting the effective temperature calibration of Chlebowski & Garmany (1991), the spectral types derived above yield $T_{\rm eff} = 37100 \pm 1000$K and $T_{\rm eff} = 38\, 100 \pm 1000$K for the primary and the secondary respectively. The quoted uncertainty corresponds to half a spectral subtype.

Howarth et al. (1997) and Penny et al. (1999) quote a luminosity ratio in the UV and in the visible of $I_{\rm sec}/I_{\rm prim} \simeq 0.98$. Raboud et al. (1997) reported V = 6.09 and E(B-V) = 0.46 for HD152248. The photometry of HD152248 is affected by the presence of a nearby visual companion that is about 2.0 mag fainter than the close binary (Mason et al. 1998). The values of the distance modulus of the NGC6231 cluster range from 10.7 to 11.6. In the following, we will adopt the value DM = 11.2 derived by Raboud et al. (1997). We thus obtain $\log{(L^{\rm prim}_{\rm bol}/L_{\odot})} = 5.61 \pm 0.2$ and $\log{(L^{\rm sec}_{\rm bol}/L_{\odot})} = 5.63 \pm 0.2$ for the primary and secondary respectively. The uncertainties on the quoted luminosities correspond to the sole uncertainty (0.5 dex) on the distance modulus of NGC6231.

The locations of the components of HD152248 are plotted in a H-R diagram in Fig. 10 together with the evolutionary tracks of Schaller et al. (1992). From the above results, we can infer radii of $R_1 = 15.4^{+3.9}_{-3.1}\,R_{\odot}$ and $R_2 = 14.9^{+3.9}_{-3.1}\,R_{\odot}$. These radii overlap within the errors with the values derived by PGB (13.4 and 12.9$R_{\odot }$ for the primary and secondary respectively) and Mayer et al. (15.6 and 17.1$R_{\odot }$ for the primary and secondary respectively). Our values of the radii are slightly larger than the typical radii of O7-O7.5 giants as listed by Howarth & Prinja (1989), but they are definitely smaller than the radii of luminosity class I stars of same spectral type (Howarth & Prinja 1989) lending further support to our assertion that the components in HD152248 are giants rather than supergiants.

From a crude interpolation between the evolutionary tracks of Schaller et al. (1992), we derive ``theoretical'' masses of $M_1 = 41.7^{+10.3}_{-6.1}\,M_{\odot}$ and $M_2 = 43.7^{+10.1}_{-6.1}\,M_{\odot}$. If we adopt the inclination of 67.2$^\circ$ as determined by Mayer et al. (2001), the minimal masses reported in Table 3 yield observed masses of 29.6 and 29.9$M_{\odot}$ for the primary and the secondary respectively. These latter values are significantly smaller than the masses predicted from the theoretical models for stars at the same location in the HR diagram. This problem was already pointed out by PGB. Improved stellar evolution models including the effects of rotation (Meynet & Maeder 2000) might provide a solution for this mass discrepancy. In fact, the new models discussed by Meynet & Maeder (2000) predict a large scatter in the mass-luminosity relation that can amount to a difference of 30% in mass. However, we caution that the components of HD152248 do not show an exceptionally large projected equatorial rotational velocity (see Howarth et al. 1997).

Figure 11: Schematic view of the wind-wind interaction process occurring within the HD152248 system. The shaded area represents the high density, emitting region. The P and S letters refer respectively to the primary and secondary components of the system. Conjunction and quadrature phases have been indicated. The adopted value for the separation between the two components is 53.1 $R_{\odot }$, corresponding to $i=67.2\hbox {$^\circ $ }$, and the shapes of the stars have been computed assuming mean radii $R_1=15.6~R_{\odot}$ and $R_2=17.1~R_{\odot}$ (Mayer et al. 2001) respectively

Another way to account for this mass discrepancy consists to postulate a Roche lobe overflow scenario that could have altered the evolution of the components. Indeed the stars are close to their stability limit near periastron passage and mass loss could be initiated at that time. However, we did not find any evidence that such a process is currently taking place in HD152248. The stars might, however, have been transferring mass through the L₁ point in the past, though the amount of transferred mass as well as the reason why this process came to a stop are unclear. Finally, we also mention Gayley's suggestion (Gayley 2001) that non-conservative mass transfer prior to the RLOF phase might exist in highly eccentric close binaries. As this is still a ``working idea'', we refer to his paper for more details.
HD 152248: Evidence for interaction