skip to main content


Title: How the breakout-limited mass in B-star centrifugal magnetospheres controls their circumstellar H α emission
ABSTRACT Strongly magnetic B-type stars with moderately rapid rotation form ‘centrifugal magnetospheres’ (CMs) from the magnetic trapping of stellar wind material in a region above the Kepler co-rotation radius. A long-standing question is whether the eventual loss of such trapped material occurs from gradual drift and/or diffusive leakage, or through sporadic ‘centrifugal breakout’ (CBO) events, wherein magnetic tension can no longer contain the built-up mass. We argue here that recent empirical results for Balmer-α emission from such B-star CMs strongly favour the CBO mechanism. Most notably, the fact that the onset of such emission depends mainly on the field strength at the Kepler radius, and is largely independent of the stellar luminosity, strongly disfavours any drift/diffusion process, for which the net mass balance would depend on the luminosity-dependent wind feeding rate. In contrast, we show that in a CBO model, the maximum confined mass in the magnetosphere is independent of this wind feeding rate and has a dependence on field strength and Kepler radius that naturally explains the empirical scalings for the onset of H α emission, its associated equivalent width, and even its line profile shapes. However, the general lack of observed Balmer emission in late-B and A-type stars could still be attributed to a residual level of diffusive or drift leakage that does not allow their much weaker winds to fill their CMs to the breakout level needed for such emission; alternatively, this might result from a transition to a metal–ion wind that lacks the requisite hydrogen.  more » « less
Award ID(s):
1663696 1716436
NSF-PAR ID:
10215543
Author(s) / Creator(s):
; ; ; ; ;
Date Published:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
499
Issue:
4
ISSN:
0035-8711
Page Range / eLocation ID:
5366 to 5378
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT

    Magnetic confinement of stellar winds leads to the formation of magnetospheres, which can be sculpted into centrifugal magnetospheres (CMs) by rotational support of the corotating plasma. The conditions required for the CMs of magnetic early B-type stars to yield detectable emission in H α – the principal diagnostic of these structures – are poorly constrained. A key reason is that no detailed study of the magnetic and rotational evolution of this population has yet been performed. Using newly determined rotational periods, modern magnetic measurements, and atmospheric parameters determined via spectroscopic modelling, we have derived fundamental parameters, dipolar oblique rotator models, and magnetospheric parameters for 56 early B-type stars. Comparison to magnetic A- and O-type stars shows that the range of surface magnetic field strength is essentially constant with stellar mass, but that the unsigned surface magnetic flux increases with mass. Both the surface magnetic dipole strength and the total magnetic flux decrease with stellar age, with the rate of flux decay apparently increasing with stellar mass. We find tentative evidence that multipolar magnetic fields may decay more rapidly than dipoles. Rotational periods increase with stellar age, as expected for a magnetic braking scenario. Without exception, all stars with H α emission originating in a CM are (1) rapid rotators, (2) strongly magnetic, and (3) young, with the latter property consistent with the observation that magnetic fields and rotation both decrease over time.

     
    more » « less
  2. ABSTRACT

    In this paper we present the first set of 3D magnetohydrodynamic (MHD) simulations performed with the riemann geomesh code. We study the dynamics of the magnetically channeled winds of magnetic massive stars in full three dimensions using a code that is uniquely suited to spherical problems. Specifically, we perform isothermal simulations of a smooth wind on a rotating star with a tilted, initially dipolar field. We compare the mass-loss, angular momentum loss, and magnetospheric dynamics of a template star (with the properties that are reminiscent of the O4 supergiant ζ Pup) over a range of rotation rates, magnetic field strengths, and magnetic tilt angles. The simulations are run up to a quasi-steady state and the results are observed to be consistent with the existing literature, showing the episodic centrifugal breakout events of the mass outflow, confined by the magnetic field loops that form the closed magnetosphere of the star. The catalogued results provide perspective on how angular-momentum loss varies for different configurations of rotation rate, magnetic field strength, and large magnetic tilt angles. In agreement with previous 2D MHD studies, we find that high magnetic confinement reduces the overall mass-loss rate, and higher rotation increases the mass-loss rate. This and future studies will be used to estimate the angular-momentum evolution, spin-down time, and mass-loss evolution of magnetic massive stars as a function of magnetic field strength, rotation rate, and dipole tilt.

     
    more » « less
  3. Abstract Asteroseismology of bright stars has become increasingly important as a method to determine the fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a revolution by detecting oscillations in more than 500 main-sequence and subgiant stars. However, most Kepler stars are faint and therefore have limited constraints from independent methods such as long-baseline interferometry. Here we present the discovery of solar-like oscillations in α Men A, a naked-eye ( V = 5.1) G7 dwarf in TESS’s southern continuous viewing zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog α Men A ( T eff = 5569 ± 62 K, R ⋆ = 0.960 ± 0.016 R ⊙ , M ⋆ = 0.964 ± 0.045 M ⊙ ). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding M ⋆ = 0.169 ± 0.006 M ⊙ , R ⋆ = 0.19 ± 0.01 R ⊙ , and T eff = 3054 ± 44 K. Our asteroseismic age of 6.2 ± 1.4 (stat) ± 0.6 (sys) Gyr for the primary places α Men B within a small population of M dwarfs with precisely measured ages. We combined multiple ground-based spectroscopy surveys to reveal an activity cycle of P = 13.1 ± 1.1 yr for α Men A, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of ∼30 days for the primary. Alpha Men A is now the closest ( d = 10 pc) solar analog with a precise asteroseismic age from space-based photometry, making it a prime target for next-generation direct-imaging missions searching for true Earth analogs. 
    more » « less
  4. ABSTRACT In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere’s co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons. 
    more » « less
  5. ABSTRACT

    We report on our combined analysis of HST, VLT/MUSE, VLT/SINFONI, and ALMA observations of the local Seyfert 2 galaxy, NGC 5728 to investigate in detail the feeding and feedback of the active galactic nucleus (AGN). The data sets simultaneously probe the morphology, excitation, and kinematics of the stars, ionized gas, and molecular gas over a large range of spatial scales (10 pc to 10 kpc). NGC 5728 contains a large stellar bar that is driving gas along prominent dust lanes to the inner 1 kpc where the gas settles into a circumnuclear ring. The ring is strongly star forming and contains a substantial population of young stars as indicated by the lowered stellar velocity dispersion and gas excitation consistent with H ii regions. We model the kinematics of the ring using the velocity field of the CO (2–1) emission and stars and find it is consistent with a rotating disc. The outer regions of the disc, where the dust lanes meet the ring, show signatures of inflow at a rate of 1 M$\odot$ yr−1. Inside the ring, we observe three molecular gas components corresponding to the circular rotation of the outer ring, a warped disc, and the nuclear stellar bar. The AGN is driving an ionized gas outflow that reaches a radius of 250 pc with a mass outflow rate of 0.08 M$\odot$ yr−1 consistent with its luminosity and scaling relations from previous studies. While we observe distinct holes in CO emission which could be signs of molecular gas removal, we find that largely the AGN is not disrupting the structure of the circumnuclear region.

     
    more » « less