SMC B-type Supergiants: Stellar winds in a low metallicity environment.

Transcription

1 Stellar Evolution at low Metallicity: Mass Loss, Eplosions, Cosmology ASP Conference Series, Vol. 353, 2006 Henny J.G.L.M. Lamers, Norbert Langer, Tiit Nugis, Kalju Annuk SMC B-type Supergiants: Stellar winds in a low metallicity environment. C.Trundle 1, D.J. Lennon 2, J. Puls 3, P.L. Dufton 4, & C.J. Evans 2 (1)Instituto de Astrofísica de Canarias, Calle Vía Lactea s/n, La Laguna, Tenerife, Spain. (2)Isaac Newton Group of Telescopes, Apartado de Correos 321, Santa Cruz de la Palma, Tenerife, Spain. (3) Universitäts-Sternwarte München, Scheinstr. 1, D-81679, Germany. (4) Dept. of Physics & Astronomy, Queen s University, Belfast, BT7 1NN, N. Ireland. Abstract. Within our Local Group the SMC provides an ideal testbed for studying stellar parameters in a low metallicity environment. Here the properties of the stellar winds of B-type supergiants in the SMC are presented, as obtained from a spectroscopic analysis. The mass-loss rates of these supergiants are compared with observational results of their galactic counterparts and theoretical predictions from monte-carlo simulations. These comparisons reveal a discrepancy between the theoretical predictions and observations, the relevance of which is discussed in terms of its influence on our understanding of stellar evolution. Introduction As the possible progenitors of some of the most extreme energy sources in the universe (viz. Wolf-Rayet stars, Supernovae, Gamma Ray Bursts etc), it is important that we understand the evolution of massive stars. In an attempt to ascertain the origins of the observed enrichments of helium and nitrogen in the photospheres of OB stars, it has become evident that two of the important factors driving the evolution of these objects are their rotation rates and the strength of their stellar winds. These two properties are intimately linked through angular momentum; the existence of a stellar wind can reduce the angular momentum of a star, which in turn causes a decrease in the rotational velocity. This connection between the mass-loss of a star and its rotational velocity affects the stellar lifetimes and photospheric composition predicted by stellar evolution codes. Thus mass-loss rates, rotational velocities and abundances are important observational constraints for such codes. The inclusion of rotationally induced mixing in stellar evolution codes such as those by Maeder & Meynet (2001), have been shown to reproduce the large dispersion of nitrogen abundances observed in B-type supergiants, however they rely heavily on the observed rotational velocity distribution and mass-loss rates. The stellar winds in massive stars are radiativelydriven, by momentum transfer by the absorption of photons in metal lines. This inevitably means that the strength of the stellar winds is dependent on the metallicity of the star. This is 127

2 128 C. Trundle et al. a crucial result of the theory of line driven winds and can be used to scale the mass-loss rates with metallicity. Stellar evolution codes such as those of Maeder & Meynet (2001), employ the relationship Ṁ(Z)=(Z/Z ) α Ṁ(Z ) where α controls the metallicity dependence. Therefore this α factor has an important role in modern day astronomy as it allows us to scale the present day mass-loss rates to those of the early universe. Through monte-carlo simulations, Vink, de Koter, & Lamers (2001) found this metallicity dependence to be 0.64 for B-type supergiants. In this paper, we compare the observed stellar wind parameters of B-type supergiants in the low metallicity environment of the SMC with theoretical predictions from montecarlo simulations by Vink, de Koter, & Lamers. We also assess the effect of metallicity on the mass-loss rates of these objects by comparing these results with observed mass-loss rates of their galactic counterparts. Wind parameters of SMC B supergiants. Seventeen B-type supergiants with spectral types ranging from B0-B5 have been analysed using the unified model atmosphere code FASTWIND (Puls, Urbaneja & Venero (2005) and references therein), to determine their stellar parameters and metallicities. These results have been presented in Trundle et al. (2004) and Trundle & Lennon (2005), where further details can be found on the determined parameters and the data used for this work, here we will concentrate on the stellar wind parameters. Figure 1. Observed mass-loss rates of Galactic and SMC mass-loss rates. The Galactic sample are from Crowther, Lennon & Walborn (2005), where as the SMC sample are from Trundle et al. (2004) and Trundle & Lennon (2005). Left: Early B-type supergiants (B0 - B1.5 with T eff > 23.5 kk). Right: Mid B-type supergiants (B1.5 - B3 with T eff < 23.5 kk).the Galactic sample are plotted with open diamonds and the SMC with solid squares. Also shown are the linear least square fits, taking into account the uncertainties of both axes.

3 Stellar Winds in the SMC. 129 In Fig. 1, the wind-momenta (product of mass-loss rate, terminal velocity and square of the radius) of these 17 SMC objects are presented, separated into two figures; one for early B type objects and the other for the mid B type stars. As previously seen for Galactic B type supergiants by Kudritzki et al. (1999), the wind momenta of mid B supergiants with T eff < 23.5kK tends to be lower that that of the hotter early B supergiants. Also presented in Fig. 1 are the results from Galactic supergiants obtained by Crowther, Lennon & Walborn (2005), which show that the Galactic supergiants, both in the case of the early and mid B supergiants have stronger wind-momenta than the SMC sample. The difference in the wind-momenta of these samples is likely to be a result of the different metallicities of their environments (i.e. Z SMC = 0.2Z Solar ). Due to the different slopes of these relationships and the scatter in the results at a given luminosity it is difficult to ascertain the exact dependence of Ṁ with Z. However the magnitude seems to be in relatively good agreement with that expected from theoretical predictions, which for this difference in metallicity is approximately a factor of 3. Over the luminosity range in common between the early B stars in the Galactic and SMC sample the wind momenta differ by dex, whilst the mid B stars differ by 0.9 to 0.5 dex stars. Figure 2. Comparison of theoretical and observed mass-loss rates of SMC targets. Theoretical predictions are calculated using the metallicity dependent mass-loss recipes of Vink, de Koter, & Lamers (2001) for Z = 0.2 Z. Left: Early B-type supergiants (B0 - B1.5 with T eff > 23.5 kk). Right: Mid B- type supergiants (B1.5 - B3 with T eff < 23.5 kk). Solid objects represent the observed mass-loss rates of the SMC objects. Also shown are the linear least square fits to these points (shown as a solid line) and the theoretical predictions (shown as a dotted line). A comparison of the wind-momenta of the observed SMC supergiants with theoretical predictions from Vink, de Koter, & Lamers (2001) is presented in Fig. 2. Here we see that the predicted wind-momenta of early-type B supergiants are lower than those observed at the metallicity of the SMC. However for the mid B-type objects, the predicted mass-loss rates are significantly higher than

4 130 C. Trundle et al. those observed. Indeed contrary to the observed datasets in both the galaxy and the SMC, the mid B-type objects have significantly higher Ṁ than the early B-type objects. Conclusions The observed SMC sample are fit by a steeper slope than the Galactic sample, (i.e the α is lower). This is as expected from theory for lower density winds Puls, Springmann, & Lennon (2000). However, whilst the theoretically predicted slopes agree well with observations for the mid B stars, they are steeper in the case of the early B supergiants. Whilst comparing the theoretical predictions with the observed results from the SMC sample, we found that for the early B stars there is a difference between the wind-momenta. This discrepancy may be eliminated by considering a certain degree of clumping in the model atmosphere codes as this would reduce the derived mass-loss rates. A larger discrepancy is seen between the observed and theoretical wind momenta of Mid B supergiants. The above solution as applied to the early B stars can not be invoked here, as such an inclusion of clumping can only exasperate the problem; increasing the separation between the theoretical predictions and observed wind momenta. A metallicity dependence is certainly observed between the two samples. However the large spread at a given point in the two datasets and the difference in their slopes hinders the exact determination of this dependence, a difference of approximately > 0.3 dex in wind-momenta is present between the samples. It is clear that to be able to scale mass-loss rates by α to metallicities representative of the early universe, we still require larger samples of objects covering a range of metallicities. Acknowledgments. This work was based on observations at the European Southern Observatory Very Large Telescopes and NTT. CT acknowledges the Department of Higher and Further Education, Training and Employment for Northern Ireland (DEFHTE) and the Dunville Scholarships fund for their financial support during this project. This project was also partially funded by the Spanish MEC through the project AYA C References Crowther, P., Lennon, D.J. & Walborn, N., 2005, A&A, accepted Kudritzki, R.-P., Puls, J., Lennon, D.J., Venn, K.A., Reetz, J., Najarro, F., McCarthy, J.K., & Herrero, A., 1999, A&A, 350, 970 Maeder, A., & Meynet, G., 2001, A&A, 373, 555 Puls, J., Springmann, U., & Lennon, M., 2000, A&AS, 141, 23 Puls, J., Urbaneja, M.A., Venero, R., & et al., 2005, A&A, 435, 669 Trundle, C., Lennon, D.J., Puls, J., & Dufton, P.L., 2004, A&A, 417, 217 Trundle, C. & Lennon, D.J., 2005, A&A, 434, 677 Vink, J.S., dekoter, A., & Lamers, H.J.G.L.M., 2001, A&A, 369, 574

5 Discussion Stellar Winds in the SMC. 131 Bomans: Do you have targets in the SMC wing? If yes, do you see differences in the derived parameters between wing and main body stars? This could be interesting not only for the metallicity of the wing, but also concerning its distance and depth structure. Trundle: 1 out of the 17 B0 B3 supergiants was located in the wing. No obvious discrepancy was observed in its parameters and the objects in the main body of SMC. Its metallicity was in agreement with the SMC main body sample. Indeed the distance may affect L, R and Ṁ determined for this object and indeed the rest of the sample contributing to the scatter in the Ṁ at given L. It would require a significant number of objects in the SMC wing to say anything about its structure or difference in distance, if any, in relation to the SMC main body.