The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. II. Analysis of 20 More Magellanic Cloud Stars and Results from the Complete Sample

By Philip Massey, J. Puls, A. Pauldrach, F. Bresolin, R. Kudritzki, T. Simon. 2005

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In order to determine the physical properties of the hottest and most luminous stars and understand how these properties change as a function of metallicity, we have analyzed HST/UV and high-S/N optical spectra of an additional 20 Magellanic Cloud stars, doubling the sample presented in the first paper in this series. Our analysis uses non-LTE line-blanketed models that include spherical extension and the hydrodynamics of the stellar wind. In addition, our data set includes FUSE observations of O VI and HST near-UV He I and He II lines to test for consistency of our derived stellar properties for a few stars. The results from the complete sample are as follows: (1) We present an effective temperature scale for O stars as a function of metallicity. We find that the SMC O3-7 dwarfs are 4000 K hotter than Galactic stars of the same spectral type. The difference is in the sense expected due to the decreased significance of line blanketing and wind blanketing at the lower metallicities that characterize the SMC. The temperature difference between the SMC and Milky Way O dwarfs decreases with decreasing temperature, becoming negligible by spectral type B0, in accord with the decreased effects of stellar winds at lower temperatures and luminosities. The temperatures of the LMC stars appear to be intermediate between that of the Milky Way and SMC, as expected based on their metallicities. Supergiants show a similar effect but are roughly 3000-4000 K cooler than dwarfs for early O stars, also with a negligible difference by B0. The giants appear to have the same effective temperature scale as dwarfs, consistent with there being little difference in the surface gravities. When we compare our scale to other recent modeling efforts, we find good agreement with some CMFGEN results, while other CMFGEN studies are discordant, although there are few individual stars in common. WM-BASIC modeling by others has resulted in significantly cooler effective temperatures than what we find, as does the recent TLUSTY/CMFGEN study of stars in the NGC 346 cluster, but our results lead to a far more coeval placement of stars in the H-R diagram for this cluster. (2) We find that the wind momentum of these stars scales with luminosity and metallicity in the ways predicted by radiatively driven wind theory, supporting the use of photospheric analyses of hot luminous stars as a distance indicator for galaxies with resolved massive star populations. (3) A comparison of the spectroscopic masses with those derived from stellar evolutionary theory shows relatively good agreement for stars with effective temperatures below 45,000 K; however, stars with higher temperatures all show a significant mass discrepancy, with the spectroscopic masses a factor of 2 or more smaller than the evolutionary masses. This problem may in part be due to unrecognized binaries in our sample, but the result suggests a possible systematic problem with the surface gravities or stellar radii derived from our models. (4) Our sample contains a large number of stars of the earliest O types, including those of the newly proposed O2 subtype. We provide the first quantitative descriptions of their defining spectral characteristics and investigate whether the new types are a legitimate extension of the effective temperature sequence. We find that the N III/N IV emission line ratio used to define the new classes does not, by itself, serve as an effective temperature indicator within a given luminosity class: there are O3.5 V stars that are as hot or hotter than O2 V stars. However, the He I/He II ratio does not fair much better for stars this hot, as we find that He I λ4471/He II λ4542, usually taken primarily as a temperature indicator, becomes sensitive to both the mass-loss rate and surface gravities for the hottest stars. This emphasizes the need to rely on all of the spectroscopic diagnostic lines, and not simply N III/N IV or even He I/He II, for these extreme objects. (5) The two stars with the most discordant radial velocities in our sample happen to be O3 "field stars," i.e., found far from the nearest OB associations. This provides the first compelling observational evidence as to the origin of the field O stars in the Magellanic Clouds, i.e., that these are classic runaway OB stars, ejected from their birthplaces.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

• Sub-surface convection zones in hot massive stars and their observable consequences (2009, background): “The mass discrepancy in massive stars is a well documented problem (see for example Herrero et al. 1992; Lennon et al. 2003; Trundle & Lennon 2005; Massey et al. 2005; Mokiem et al. 2007).”
• Mass loss from hot massive stars (2008, background): “(iv) For a given luminosity, the mass-loss rates of SMC-stars are lower than for their Galactic counterparts, consistent with theory (Massey et al 2004, 2005).”
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars (2007, methodology): “Probably its discrepant position can be explained by the fact that Massey et al. (2005) could only determine a lower limit for its effective temperature, hence it could be intrinsically brighter.”
• The VLT-FLAMES survey of massive stars: evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds (2007, background): “17 line blanketing and non-LTE effects in the model atmospheres (Martins et al. 2002; Crowther et al. 2002, 2006; Massey et al. 2004, 2005).”
• The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II λ1640 in young star clusters (2016, methodology): “Since then, a number of HST programmes have obtained UV spectroscopy for a range of LMC O stars (e.g. Massey et al. 2004, 2005, 2009).”
• The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars (2009, result): “…derived temperature scales for Galactic and Magellanic Cloud stars, which imply that stars at lower metallicity have higher effective temperatures for a given spectral type, in broad agreement with previous studies (Martins et al. 2002, 2005; Crowther et al. 2002, 2006; Massey et al. 2004, 2005).”
• The VLT-FLAMES survey of massive stars: surface chemical compositions of B-type stars in the Magellanic Clouds (2006, background): “In previous studies, see for example Voels et al. (1989) and Massey et al. (2005), it has been reported that there is a significant difference in temperature scales between different luminosity class stars at the same metallicity and also for stars of the same spectral type at different…”
• STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE (2014, background): “It is known that early O-type Magellanic Cloud stars of a given spectral classification have Teff higher by several thousand K compared to their Galactic counterparts (e.g., Massey et al. 2005).”
• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit (2010, methodology): “Spectral types are taken from Crowther & Dessart (1998) or Walborn & Blades (1997), although other authors prefer alternative nomenclature, e.g. O2 If for both Mk 39 and R136a5 according to Massey et al. (2004, 2005).”
• The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region (2006, background): “…and NGC 330 in the SMC, and on NGC 2004 and the N11 region (Henize 1956) in the LMC. NGC 346 is a young cluster with an age in the range of 1 to 3 × 106 yrs (Kudritzki et al. 1989; Walborn et al. 2000; Bouret et al. 2003; Massey et al. 2005), that has clearly undergone prodigious star formation.”
• SPITZER SAGE INFRARED PHOTOMETRY OF MASSIVE STARS IN THE LARGE MAGELLANIC CLOUD (2009, unknown): “…(M95) Massey et al. 1995; (M00) Massey et al. 2000; (M02) Massey 2002; (M02b) Massey et al. 2002; (M03) Massey 2003; (M04) Massey et al. 2004; (M05) Massey et al. 2005; (M09) Massey 2009; (Me05) Meynadier et al. 2005; (N94) Niemela & Bassino 1994; (N01) Niemela et al. 2001; (O95) Oey & Massey…”
• The Effective Temperatures and Physical Properties of Magellanic Cloud Red Supergiants: The Effects of Metallicity (2006, background): “Massey et al. (2005a) argue that this is a natural consequence of the fact that these stars are ‘‘smoky’’ and produce circumstellar dust.”
• The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class (2014, methodology): “A lower temperature limit of T∗ > 42 kK was derived by Massey et al. (2005) with FASTWIND.”
• EXTRAGALACTIC CHEMICAL ABUNDANCES: DO H ii REGIONS AND YOUNG STARS TELL THE SAME STORY? THE CASE OF THE SPIRAL GALAXY NGC 300 (2009, methodology): “Massey et al. (2004, 2005), comparing spectra of O stars located in the Milky Way and the Magellanic Clouds, and analyzed with NLTE, line-blanketed model atmospheres, find that early O stars in the SMC are 3000-4000 K hotter than their counterparts in the Milky Way.”