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Register a new userClassical Oe Stars in the Field of the Small Magellanic Cloud
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Authors
Jesse B. Golden-Marx
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We present $29pm1$ classical Oe stars from RIOTS4, a spatially complete, spectroscopic survey of Small Magellanic Cloud (SMC) field OB stars. The two earliest are O6e stars, and four are earlier than any Milky Way (MW) Oe stars. We also find ten Ope stars, showing He~textsc{i} infill and/or emission; five appear to be at least as hot as $sim$O7.5e stars. The hottest, star 77616, shows He~textsc{ii} disk emission, suggesting that even the hottest O stars can form decretion disks, and offers observational support for theoretical predictions that the hottest, fastest rotators can generate He$^+$-ionizing atmospheres. Our data also demonstrate that Ope stars correspond to Oe stars earlier than O7.5e with strong disk emission. We find that in the SMC, Oe stars extend to earlier spectral types than in the MW, and our SMC Oe/O frequency, $0.26pm0.04$, is much greater than the MW value, $0.03pm0.01$. These results are consistent with angular momentum transport by stronger winds suppressing decretion disk formation at higher metallicity. In addition, our SMC field Oe star frequency is indistinguishable from that for clusters, which is consistent with the similarity between rotation rates in these environments, and contrary to the pattern for MW rotation rates. Thus, our findings strongly support the viscous decretion disk model and confirm that Oe stars are the high-mass extension of the Be phenomenon. Additionally, we find that Fe~textsc{ii} emission occurs among Oe stars later than O7.5e with massive disks, and we revise a photometric criterion for identifying Oe stars to $J-[3.6] geq 0.1$.
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No classical Oe/Be stars with spectral type earlier than O7.5e have been identified to date in the Milky Way (MW). This is consistent with the decretion disk model because strong stellar winds cause early-type O stars to lose angular momentum, thereby preventing them from rotating fast enough to spin out decretion disks. How- ever, metal-poor O stars have weaker stellar winds, allowing the stars to retain angular momentum. Therefore, low-metallicity environments should promote the formation of Oe stars, including those of earlier spectral types than observed in high-metallicity en- vironments. Using the RIOTS4 survey, a spatially complete spectroscopic survey of Small Magellanic Cloud (SMC) field OB stars taken with the IMACS multi-slit spec- trograph at the Magellan Baade Telescope, we identify 25-31 SMC field Oe stars, which account for 20-28% of SMC field O stars. This fraction is significantly higher than in the MW, where < 10-15% of O stars display the Be phenomenon. We also present 5-7 Oe stars of spectral type ranging from O5.5e to O7e, all earlier spectral types than the earliest MW Oe star. These early type Oe stars represent 20-23% of our SMC Oe stars, a dramatic increase compared to the MW, where no Oe stars have been identified with these early spectral types. Thus, the higher frequencies of Oe stars and their earlier spectral range in the metal-poor SMC are consistent with the decretion disk model.
The Optical Gravitational Lensing Experiment identified over 1,800 carbon-rich Mira and semi-regular variables in the Small Magellanic Cloud. Multi-epoch infrared photometry reveals that the semi-regulars and Miras follow different sequences in color-color space when using colors sensitive to molecular absorption bands. The dustiest Miras have the strongest pulsation amplitudes and longest periods. Efforts to determine bolometric magnitudes reveal possible systematic errors with published bolometric corrections.
Some theories of star formation suggest massive stars may only form in clustered environments, which would create a deficit of massive stars in low density environments. Observationally, Massey (2002) finds such a deficit in samples of the field population in the Small and Large Magellanic Clouds, with an IMF slope of {Gamma} ~ 4. These IMF measurements represent some of the largest known deviations from the standard Salpeter IMF slope of {Gamma}=1.35. Here, we carry out a comprehensive investigation of the mass function above 20 solar masses for the entire field population of the Small Magellanic Cloud, based on data from the Runaways and Isolated O Type Star Spectroscopic Survey of the SMC (RIOTS4). This is a spatially complete census of the entire field OB star population of the SMC obtained with the IMACS multi-object spectrograph and MIKE echelle spectrograph on the Magellan telescopes. Based on Monte-Carlo simulations of the evolved present-day mass function, we find the slope of the field IMF above 20 solar masses is {Gamma}=2.3+/-0.4. We extend our IMF measurement to lower masses using BV photometry from the OGLE II survey. We use a statistical approach to generate a probability distribution for the mass of each star from the OGLE photometry, and we again find {Gamma}=2.3+/-0.6 for stellar masses from 7-20 solar masses. The discovery and removal of ten runaways in our RIOTS4 sample steepens the field IMF slope to {Gamma}=2.8+/-0.5. We discuss the possible effects of binarity and star-formation history on our results, and conclude that the steep field massive star IMF is most likely a real effect.
Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to 10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past.
Using archival Spitzer Space Telescope data, we identified for the first time a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the detection of their bow shocks. The geometry of detected bow shocks allows us to infer the direction of motion of the associated stars and to determine their possible parent clusters and associations. One of the identified runaway stars, AzV 471, was already known as a high-velocity star on the basis of its high peculiar radial velocity, which is offset by ~40 km/s from the local systemic velocity. We discuss implications of our findings for the problem of the origin of field OB stars. Several of the bow shock-producing stars are found in the confines of associations, suggesting that these may be alien stars contributing to the age spread observed for some young stellar systems. We also report the discovery of a kidney-shaped nebula attached to the early WN-type star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure created owing to the interaction between the stellar wind and the ambient interstellar medium.
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