Outflowing disk formation in B[e] supergiants due to rotation and bi-stability in radiation driven winds

Arias

et al. 2016

A&A

Self Cite

Context. B[e] supergiants are evolved massive stars, enshrouded in a dense wind and surrounded by a molecular and dusty disk. The mechanisms that drive phases of enhanced mass loss and mass ejections, responsible for the shaping of the circumstellar material of these objects, are still unclear. Aims. We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the Large Magellanic Cloud B[e] supergiant LHA 120-S 73. Methods. High-resolution optical and near-infrared spectroscopic data were obtained over a period of 16 and 7 yr, respectively. The spectra cover the diagnostic emission lines from [Ca ii] and [O i], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A lowresolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk. Results. All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that the profile of each forbidden line contains contributions from two spatially clearly distinct rings. In total, we find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly nonmonotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The optical spectra additionally display a broad emission feature at 6160-6180 Å, which we interpret as molecular emission from TiO. The mid-infrared spectrum displays features of oxygen-and carbon-rich grain species, which indicates a long-lived, stable dusty disk.We cannot confirm the previously reported high value for the stellar rotation velocity. He i λ 5876 is the only clearly detectable pure atmospheric absorption line in our data. Its line profile is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible. Conclusions. The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.

Section: Origin Of the Circumstellar Ringsmentioning

confidence: 86%

Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73

Kraus

Arias

et al. 2016

A&A

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“…This effect would change the ionization structure of the flow with latitude, leading to different wind regimes that favor the formation of outflowing disk or kink structures. Models based on this assumption were discussed by Zickgraf et al (1985), Lamers & Pauldrach (1991), and Curé et al (2005). In the last two works, the authors also analyzed the effects of the bi-stability temperature jump as a mechanism to produce a switch in the wind regime at a given latitude.…”

Section: Physical Properties and Applications Of The Modelsmentioning

confidence: 99%

The Wind of Rotating B Supergiants. I. Domains of Slow and Fast Solution Regimes

Venero

Curé

et al. 2016

ApJ

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In the scenario of rotating radiation-driven wind theory for massive stars, three types of stationary hydrodynamic solutions are currently known: the classical ( fast) m-CAK solution, the Ω-slow solution that arises for fast rotators, and the so-called δ-slow solution if high values of the δ line-force parameter are allowed independently of the rotation speed. Compared to the fast solution, both "slow solutions" have lower terminal velocities. As the study of the parameter domain for the slow solution is still incomplete, we perform a comprehensive analysis of the distinctive flow regimes for B supergiants that emerge from a fine grid of rotation values, Ω, and various ionization conditions in the wind(δ) parameter. The wind ionization defines two domains: one for fast outflowing winds and the other for slow expanding flows. Both domains are clear-cut by a gap, where a kink/plateau structure of the velocity law could exist for a finite interval of δ. The location and width of the gap depend on T eff and Ω. There is a smooth and continuous transition between the Ω-slow and δ-slow regimes, a single Ω δ-slow regime. We discuss different situations where the slow solutions can be found and the possibility of a switch between fast and slow solutions in B supergiant winds. We compare the theoretical terminal velocity with observations of B and A supergiants and find that the fast regime prevails mostly for early B supergiants while the slow wind regime matches better for A and B mid-and late-type supergiants.

“…B[e]SGs show various observational signatures of a dense circumstellar disk or ring, including narrow forbidden emission lines, molecular emission, and evidence for dust (for a general review see Lamers et al 1998). Zickgraf et al (1985) proposed a disk formation mechanism for B[e] stars, in which rapid rotation produces a hybrid wind consisting of a fast line-driven polar component and a slow, high density equatorial component (with further study by, e.g., Lamers & Pauldrach 1991;Pelupessy et al 2000;Kraus & Lamers 2003;Curé 2004;Curé & Rial 2004;Curé et al 2005;Kraus 2006). While the spectral features certainly support this process, rotational speeds have been determined for only a few B[e]SGs including LHA 115-S 23 (Kraus et al 2008), LHA 115-S 65 (Zickgraf 2000;Kraus et al 2010), LHA 120-S 93 (Gummersbach et al 1995), and LHA 120-S 73 (Zickgraf 2006).…”

Section: Introductionmentioning

confidence: 99%

Probing the ejecta of evolved massive stars in transition

Oksala

Kraus

et al. 2013

A&A

139

Massive evolved stars in transition phases, such as luminous blue variables (LBVs), B[e] supergiants (B[e]SGs), and yellow hypergiants (YHGs), are not well understood, and yet crucial steps in determining accurate stellar and galactic evolution models. The circumstellar environments of these stars reveal their mass-loss history, identifying clues to both their individual evolutionary status and the connection between objects of different phases. Here we present a survey of 25 such evolved massive stars (16 B[e]SGs, 6 LBVs, 2 YHGs, and 1 Peculiar Oe star), observed in the K-band with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 8 m telescope. The sample can be split into two categories based on spectral morphology: one group includes all of the B[e]SGs, the Peculiar Oe star, and two of the LBVs, while the other includes the YHGs and the rest of the LBVs. The difference in LBV spectral appearance is due to some objects being in a quiescent phase and some objects being in an active or outburst phase. CO emission features are found in 13 of our targets, with first time detections for MWC 137, LHA 120-S 35, and LHA 115-S 65. From model fits to the CO band heads, the emitting regions appear to be detached from the stellar surface. Each star with 12 CO features also shows 13 CO emission, signaling an evolved nature. Based on the level of 13 C enrichment, we conclude that many of the B[e]SGs are likely in a pre-Red Supergiant phase of their evolution. There appears to be a lower luminosity limit of log L/L = 5.0 below which CO is not detected. The lack of CO features in several high luminosity B[e]SGs and variability in others suggests that they may in fact be LBV candidates, strengthening the connection between these two very similar transition phases.