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Register a new userConfirming HD 23478 as a new magnetic B star hosting an H$alpha$-bright centrifugal magnetosphere
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In this paper we report 23 magnetic field measurements of the B3IV star HD 23478: 12 obtained from high resolution Stokes $V$ spectra using the ESPaDOnS (CFHT) and Narval (TBL) spectropolarimeters, and 11 from medium resolution Stokes $V$ spectra obtained with the DimaPol spectropolarimeter (DAO). HD 23478 was one of two rapidly rotating stars identified as potential centrifugal magnetosphere hosts based on IR observations from the Apache Point Observatory Galactic Evolution Experiment survey. We derive basic physical properties of this star including its mass ($M=6.1^{+0.8}_{-0.7},M_odot$), effective temperature ($T_{rm eff}=20pm2,$kK), radius ($R=2.7^{+1.6}_{-0.9},R_odot$), and age ($tau_{rm age}=3^{+37}_{-1},$Myr). We repeatedly detect weakly-variable Zeeman signatures in metal, He and H lines in all our observations corresponding to a longitudinal magnetic field of $langle B_zrangleapprox-2.0,$kG. The rotational period is inferred from Hipparcos photometry ($P_{rm rot}=1.0498(4),$d). Under the assumption of the Oblique Rotator Model, our obsevations yield a surface dipole magnetic field of strength $B_dgeq9.5,$kG that is approximately aligned with the stellar rotation axis. We confirm the presence of strong and broad H$alpha$ emission and gauge the volume of this stars centrifugal magnetosphere to be consistent with those of other H$alpha$ emitting centrifugal magnetosphere stars based on the large inferred Alfven to Kepler radius ratio.
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The SDSS III APOGEE survey recently identified two new $sigma$ Ori E type candidates, HD 345439 and HD 23478, which are a rare subset of rapidly rotating massive stars whose large (kGauss) magnetic fields confine circumstellar material around these systems. Our analysis of multi-epoch photometric observations of HD 345439 from the KELT, SuperWASP, and ASAS surveys reveals the presence of a $sim$0.7701 day period in each dataset, suggesting the system is amongst the faster known $sigma$ Ori E analogs. We also see clear evidence that the strength of H-alpha, H I Brackett series lines, and He I lines also vary on a $sim$0.7701 day period from our analysis of multi-epoch, multi-wavelength spectroscopic monitoring of the system from the APO 3.5m telescope. We trace the evolution of select emission line profiles in the system, and observe coherent line profile variability in both optical and infrared H I lines, as expected for rigidly rotating magnetosphere stars. We also analyze the evolution of the H I Br-11 line strength and line profile in multi-epoch observations of HD 23478 from the SDSS-III APOGEE instrument. The observed periodic behavior is consistent with that recently reported by Sikora and collaborators in optical spectra.
Rapidly rotating early-type stars with strong magnetic fields frequently show H$alpha$ emission originating in Centrifugal Magnetospheres (CMs), circumstellar structures in which centrifugal support due to magnetically enforced corotation of the magnetically confined plasma enables it to accumulate to high densities. It is not currently known whether the CM plasma escapes via Centrifugal Breakout (CB), or by an unidentified leakage mechanism. We have conducted the first comprehensive examination of the H$alpha$ emission properties of all stars currently known to display CM-pattern emission. We find that the onset of emission is dependent primarily on the area of the CM, which can be predicted simply by the value $B_{rm K}$ of the magnetic field at the Kepler corotation radius $R_{rm K}$. Emission strength is strongly sensitive to both CM area and $B_{rm K}$. Emission onset and strength are {em not} dependent on effective temperature, luminosity, or mass-loss rate. These results all favour a CB scenario, however the lack of intrinsic variability in any CM diagnostics indicates that CB must be an essentially continuous process, i.e. it effectively acts as a leakage mechanism. We also show that the emission profile shapes are approximately scale-invariant, i.e. they are broadly similar across a wide range of emission strengths and stellar parameters. While the radius of maximum emission correlates closely as expected to $R_{rm K}$, it is always larger, contradicting models that predict that emission should peak at $R_{rm K}$.
We report the results of 6 nights of Canada-France-Hawaii Telescope spectropolarimetric ESPaDOnS observations of the He-strong, magnetic B1 type star ALS 3694. The longitudinal magnetic field is approximately 2 kG in all 6 observations, showing essentially no variation between nights. The H$alpha$ line displays variable emission on all nights, peaking at high velocities ($sim 3 vsin{i}$). Given the presence of a strong ($B_{rm d}>$6 kG) magnetic field, and the similarity of the emission profile to that of other magnetic B-type stars, we interpret the emission as a consequence of a centrifugal magnetosphere.
W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large dataset of high-resolution spectropolarimetry. The star is a previously unreported spectroscopic binary, consisting of 2 B2 stars with a mass ratio of 1.8, masses of 12 M$_odot$ and 6.2 $M_odot$, in an eccentric 110-day orbit. The magnetic field belongs to the secondary, W 601 B. The H$alpha$ emission is consistent with an origin in W 601 Bs centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disk. However, the low value of $log{g} = 3.8$ determined via spectroscopic analysis, and the stars membership in the young NGC 6611 cluster, are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the H$alpha$ line and the longitudinal magnetic field $langle B_z rangle$ is 1.13 d. Modelling of Stokes $V$ and $langle B_z rangle$ indicates a surface dipolar magnetic field $B_{rm d}$ between 6 and $11$ kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to H$alpha$-bright magnetic B-type stars such as $sigma$ Ori E. By contrast, the primary is an apparently non-magnetic ($B_{rm d} < 300$ G) pre-main sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.
HD 179949 is an F8V star, orbited by a giant planet at ~8 R* every 3.092514 days. The system was reported to undergo episodes of stellar activity enhancement modulated by the orbital period, interpreted as caused by Star-Planet Interactions (SPIs). One possible cause of SPIs is the large-scale magnetic field of the host star in which the close-in giant planet orbits. In this paper we present spectropolarimetric observations of HD 179949 during two observing campaigns (2009 September and 2007 June). We detect a weak large-scale magnetic field of a few Gauss at the surface of the star. The field configuration is mainly poloidal at both observing epochs. The star is found to rotate differentially, with a surface rotation shear of dOmega=0.216pm0.061 rad/d, corresponding to equatorial and polar rotation periods of 7.62pm0.07 and 10.3pm0.8 d respectively. The coronal field estimated by extrapolating the surface maps resembles a dipole tilted at ~70 degrees. We also find that the chromospheric activity of HD 179949 is mainly modulated by the rotation of the star, with two clear maxima per rotation period as expected from a highly tilted magnetosphere. In September 2009, we find that the activity of HD 179949 shows hints of low amplitude fluctuations with a period close to the beat period of the system.
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