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Register a new userThe B[e] phenomenon in the Milky Way and Magellanic Clouds
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Added by
Anatoly Miroshnichenko
Publication date
2010
fields
Physics
and research's language is
English
Authors
Anatoly Miroshnichenko
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Discovered over 30 years ago, the B[e] phenomenon has not yet revealed all its puzzles. New objects that exhibit it are being discovered in the Milky Way, and properties of known objects are being constrained. We review recent findings about objects of this class and their subgroups as well as discuss new results from studies of the objects with yet unknown nature. In the Magellanic Clouds, the population of such objects has been restricted to supergiants. We present new candidates with apparently lower luminosities found in the LMC.
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B[e] supergiants are evolved massive stars with a complex circumstellar environment. A number of important emission features probe the structure and the kinematics of the circumstellar material. In our survey of Magellanic Cloud B[e] supergiants we focus on the [OI] and [CaII] emission lines, which we identified in four more objects.
In order to understand the rates and properties of Type Ia and Type Ib/c supernovae, X-ray binaries, gravitational wave sources, and gamma ray bursts as a function of galactic environment and cosmic age, it is imperative that we measure how the close binary properties of O and B-type stars vary with metallicity. We have studied eclipsing binaries with early-B main-sequence primaries in three galaxies with different metallicities: the Large and Small Magellanic Clouds (LMC and SMC, respectively) as well as the Milky Way (MW). The observed fractions of early-B stars which exhibit deep eclipses 0.25 < Delta(m) (mag) < 0.65 and orbital periods 2 < P (days) < 20 in the MW, LMC, and SMC span a narrow range of (0.7-1.0)%, which is a model independent result. After correcting for geometrical selection effects and incompleteness toward low-mass companions, we find for early-B stars in all three environments: (1) a close binary fraction of (22+/-5)% across orbital periods 2 < P (days) < 20 and mass ratios q = M_2/M_1 > 0.1, (2) an intrinsic orbital period distribution slightly skewed toward shorter periods relative to a distribution that is uniform in log P, (3) a mass-ratio distribution weighted toward low-mass companions, and (4) a small, nearly negligible excess fraction of twins with q > 0.9. Our fitted parameters derived for the MW eclipsing binaries match the properties inferred from nearby, early-type spectroscopic binaries, which further validates our results. There are no statistically significant trends with metallicity, demonstrating that the close binary properties of massive stars do not vary across metallicities -0.7 < log(Z/Z_sun) < 0.0 beyond the measured uncertainties.
We have measured the elemental abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra in order to establish the influence of the chemical composition on the properties of these stars (see Romaniello et al. 2005). Here we describe the robust analytical procedure we have developed to accurately determine them. The resulting iron abundances span a range between ~ -0.80 dex for stars in the Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the Galaxy. While the average values for each galaxy are in good agreement with non-pulsating stars of similar age, Cepheids display a significant spread. Thus it is fundamental to measure the metallicity of individual stars.
Diffuse interstellar bands (DIBs) trace warm neutral and weakly-ionized diffuse interstellar medium (ISM). Here we present a dedicated, high signal-to-noise spectroscopic study of two of the strongest DIBs, at 5780 and 5797 AA, in optical spectra of 666 early-type stars in the Small and Large Magellanic Clouds, along with measurements of the atomic Na,{sc i},D and Ca,{sc ii},K lines. The resulting maps show for the first time the distribution of DIB carriers across large swathes of galaxies, as well as the foreground Milky Way ISM. We confirm the association of the 5797 AA DIB with neutral gas, and the 5780 AA DIB with more translucent gas, generally tracing the star-forming regions within the Magellanic Clouds. Likewise, the Na,{sc i},D line traces the denser ISM whereas the Ca,{sc ii},K line traces the more diffuse, warmer gas. The Ca,{sc ii},K line has an additional component at $sim200$--220 km s$^{-1}$ seen towards both Magellanic Clouds; this may be associated with a pan-Magellanic halo. Both the atomic lines and DIBs show sub-pc-scale structure in the Galactic foreground absorption; the 5780 and 5797 AA DIBs show very little correlation on these small scales, as do the Ca,{sc ii},K and Na,{sc i},D lines. This suggests that good correlations between the 5780 and 5797 AA DIBs, or between Ca,{sc ii},K and Na,{sc i},D, arise from the superposition of multiple interstellar structures. Similarity in behaviour between DIBs and Na,{sc i} in the SMC, LMC and Milky Way suggests the abundance of DIB carriers scales in proportion to metallicity.
Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances >200 kpc (i.e., Mvir > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends roughly 100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.
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