Abstract Digest #2, 28.7.2014

The BeSN is introducing a new model of distributing abstracts. Instead of relying on abstracts being sent directly by the authors, everyone is invited to submit abstracts and short bits of news of interest, which then will be published regularly in a digest. Today's abstracts:

  • "Giant Outbursts in Be/X-Ray Binaries" by R.G. Martin et al.
  • "A Strict Test of Stellar Evolution Models: The Absolute Dimensions of the Massive Benchmark Eclipsing Binary V578 Mon" by E.V. Garcia et al.
  • "ALMA Hints at the Presence of two Companions in the Disk around HD 100546" by C. Walsh et al.
  • "On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars" by C. Aerts et al.
  • "Massive Star Asteroseismology in Action" by C. Aerts
  • "Asteroseismic Diagnostics for Semi-Convection in B Stars in the Era of K2" by E. Moraweji
  • "Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis" by A. David-Uraz et al.

Giant Outbursts in Be/X-Ray Binaries

R.G. Martin et al.

Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and giant (Type II) outbursts, whose origins have remained elusive. We suggest that Type II X-ray outbursts occur when a highly misaligned decretion disk around the Be star becomes eccentric, allowing the compact object companion to capture a large amount of material at periastron. Using three-dimensional smoothed particle hydrodynamics simulations, we model the long-term evolution of a representative Be/X-ray binary system. We find that periodic (Type I) X-ray outbursts occur when the neutron star is close to periastron for all disk inclinations. Type II outbursts occur for large misalignment angles and are associated with eccentricity growth which occurs on a timescale of about 10 orbital periods. Mass capture from the eccentric decretion disk results in an accretion disk around the neutron star whose estimated viscous time is long enough to explain the extended duration of Type II outbursts. Previous studies suggested that the outbursts are caused by a warped disk but our results suggest that this is not sufficient; the disk must be both highly misaligned and eccentric to initiate a Type II accretion event.

Available at: ApJ 790, L34


A Strict Test of Stellar Evolution Models: The Absolute Dimensions of the Massive Benchmark Eclipsing Binary V578 Mon

E.V. Garcia et al.

We determine the absolute dimensions of the eclipsing binary V578 Mon, a detached system of two early B-type stars (B0V + B1V, P = 2.40848 days) in the star-forming region NGC 2244 of the Rosette Nebula. From the light curve analysis of 40 yr of photometry and the analysis of HERMES spectra, we find radii of 5.41 ± 0.04 solar radii and 4.29 ± 0.05 solar radii, and temperatures of 30,000 ± 500 K and 25,750 ± 435 K, respectively. We find that our disentangled component spectra for V578 Mon agree well with previous spectral disentangling from the literature. We also reconfirm the previous spectroscopic orbit of V578 Mon finding that masses of 14.54 ± 0.08 solar masses and 10.29 ± 0.06 solar masses are fully compatible with the new analysis. We compare the absolute dimensions to the rotating models of the Geneva and Utrecht groups and the models of the Granada group. We find that all three sets of models marginally reproduce the absolute dimensions of both stars with a common age within the uncertainty for gravity-effective temperature isochrones. However, there are some apparent age discrepancies for the corresponding mass-radius isochrones. Models with larger convective overshoot, >0.35, worked best. Combined with our previously determined apsidal motion of 0.07089 +0.00021/-0.00013 deg/cycle, we compute the internal structure constants (tidal Love number) for the Newtonian and general relativistic contribution to the apsidal motion as log k_2 = –1.975 ± 0.017 and log k_2 = –3.412 ± 0.018, respectively. We find the relativistic contribution to the apsidal motion to be small, <4%. We find that the prediction of log k_2, theo = –2.005 ± 0.025 of the Granada models fully agrees with our observed log k_2.

Available at: AJ 148, 39


ALMA Hints at the Presence of two Companions in the Disk around HD 100546

C. Walsh et al.

HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present Atacama Large Millimeter/Submillimeter Array observations of the HD 100546 disk which resolve the gas and dust structure at (sub)millimeter wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390 ± 20 AU in radius) whereas the continuum emission is more compact (230 ± 20 AU in radius), suggesting radial drift of the millimeter-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and micrometer-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75 ± 3 AU centered at 190 ± 3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of millimeter-sized grains within a compact ring, approx 20-30 AU, by approx 10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (approx 1.2 Myr) and also triggers the development of an outer ring (approx 100-200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk.

Available at: ApJ 791, L6


On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars

C. Aerts et al.

The Fourier Transform method is a popular tool to derive the rotational velocities of stars from their spectral line profiles. However, its domain of validity does not include line-profile variables with time-dependent profiles. We investigate the performance of the method for such cases, by interpreting the line-profile variations of spotted B stars, and of pulsating B tars, as if their spectral lines were caused by uniform surface rotation along with macroturbulence. We perform time-series analysis and harmonic least-squares fitting of various line diagnostics and of the outcome of several implementations of the Fourier Transform method. We find that the projected rotational velocities derived from the Fourier Transform vary appreciably during the pulsation cycle whenever the pulsational and rotational velocity fields are of similar magnitude. The macroturbulent velocities derived while ignoring the pulsations can vary with tens of km/s during the pulsation cycle. The temporal behaviour of the deduced rotational and macroturbulent velocities are in antiphase with each other. The rotational velocity is in phase with the second moment of the line profiles. The application of the Fourier method to stars with considerable pulsational line broadening may lead to an appreciable spread in the values of the rotation velocity, and, by implication, of the deduced value of the macroturbulence. These two quantities should therefore not be derived from single snapshot spectra if the aim is to use them as a solid diagnostic for the evaluation of stellar evolution models of slow to moderate rotators.

Available at: arXiv:1407.6611


Massive Star Asteroseismology in Action

C. Aerts

After highlighting the principle and power of asteroseismology for stellar physics, we briefly emphasize some recent progress in this research for various types of stars. We give an overview of high-precision high duty-cycle space photometry of OB-type stars. Further, we update the overview of seismic estimates of stellar parameters of OB dwarfs, with specific emphasis on convective core overshoot. We discuss connections between pulsational, rotational, and magnetic variability of massive stars and end with future prospects for asteroseismology of evolved OB stars.

Available at: arXiv:1407.6479


Asteroseismic Diagnostics for Semi-Convection in B Stars in the Era of K2

E. Moraweji

Semi-convection is a slow mixing process in chemically-inhomogeneous radiative interiors of stars. In massive OB stars, it is important during the main sequence. However, the efficiency of this mixing mechanism is not properly gauged yet. Here, we argue that asteroseismology of beta Cep pulsators is capable of distinguishing between models of varying semi-convection efficiencies. We address this in the light of upcoming high-precision space photometry to be obtained with the Kepler two-wheel mission for massive stars along the ecliptic.

Available at: arXiv:1407.6536


Investigating the origin of cyclical wind variability in hot, massive stars - I. On the dipolar magnetic field hypothesis

A. David-Uraz et al.

OB stars exhibit various types of spectral variability associated with wind structures, including the apparently ubiquitous discrete absorption components (DACs). These are proposed to be caused by either magnetic fields or non-radial pulsations (NRPs). In this paper, we evaluate the possible relation between large-scale, dipolar magnetic fields and the DAC phenomenon by investigating the magnetic properties of a sample of 13 OB stars exhibiting well-documented DAC behaviour. Using high-precision spectropolarimetric data acquired in part in the context of the Magnetism in Massive Stars (MiMeS) project, we find no evidence for surface dipolar magnetic fields in any of these stars. Using Bayesian inference, we compute upper limits on the strengths of the fields and use these limits to assess two potential mechanisms by which the field may influence wind outflow: magnetic wind confinement and local photospheric brightness enhancements. Within the limits we derive, both mechanisms fail to provide a systematic process capable of producing DACs in all of the stars of our sample. Therefore, this implies that dipolar fields are highly unlikely to be responsible for these structures in all massive stars, meaning that some other mechanism must come into play.

Available at: arXiv:1407.6417


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