Dag Evensberget

• Cotton, Daniel V; Bailey, Jeremy; Kedziora-Chudczer, Lucyna; Bott, Kimberly; De Horta, Ain Y & Filcek, Normandy [Show all 17 contributors for this article] (2024). Polarization position angle standard stars: a reassessment of θ and its variability for seventeen stars based on a decade of observations. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 535(2), p. 1615–1615. doi: 10.1093/mnras/stae2418. Full text in Research Archive Show summary

  ABSTRACT Observations of polarization position angle ($\theta$) standards made from 2014 to 2023 with the High Precision Polarimetric Instrument (HIPPI) and other HIPPI-class polarimeters in both hemispheres are used to investigate their variability. Multiband data were first used to thoroughly recalibrate the instrument performance by bench-marking against carefully selected literature data. A novel co-ordinate difference matrix (CDM) approach – which combines pairs of points – was then used to amalgamate monochromatic ($g^\prime$ band) observations from many observing runs and re-determine $\theta$ for 17 standard stars. The CDM algorithm was then integrated into a fitting routine and used to establish the impact of stellar variability on the measured position angle scatter. The approach yields variability detections for stars on long time-scales that appear stable over short runs. The best position angle standards are $\ell$ Car, o Sco, HD 154445, HD 161056, and $\iota ^1$ Sco, which are stable to $\le$0.123$^\circ$. Position angle variability of 0.27–0.82$^\circ$, significant at the 3$\sigma$ level, is found for 5 standards, including the Luminous Blue Variable HD 160529 and all but one of the other B/A-type supergiants (HD 80558, HD 111613, HD 183143, and 55 Cyg), most of which also appear likely to be variable in polarization magnitude (p) – there is no preferred orientation for the polarization in these objects, which are all classified as $\alpha$ Cygni variables. Despite this we make six key recommendations for observers – relating to data acquisition, processing and reporting – that will allow them to use these standards to achieve $\lt $ 0.1$^\circ$ precision in the telescope position angle with similar instrumentation, and allow data sets to be combined more accurately.

• Strickert, K M; Evensberget, D & Vidotto, A A (2024). High-latitude coronal mass ejections on the young solar-like star AB Dor. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 533(1), p. 1165–1165. doi: 10.1093/mnras/stae1884. Full text in Research Archive Show summary

  ABSTRACT AB Dor is a young solar-type star with a surface large-scale magnetic field $10^2$ to $10^3$ times stronger than the that of the Sun. Although strong magnetic fields are thought to inhibit coronal mass ejections (CMEs), dimming signatures typically associated with an eruptive CME were recently observed in AB Dor. The uninterrupted, long-duration dimming signal suggests that a CME took place at a high latitude, where it remained in view as the star rotates. A high-latitude CME is also consistent with observations that indicate that AB Dor hosts polar active regions. To investigate magnetic confinement in AB Dor, we conduct a parametric modelling study of 21 CMEs at latitudes ${\sim} 60^\circ$, varying the location, mass, and magnetic field strength of an injected flux rope. 12 models had the flux rope located in an open magnetic field region, while the remaining nine were in a closed region. Results show that CMEs in open-field regions are in general more likely to erupt. The four eruptive CMEs from closed regions had high free magnetic energies ${\gtrsim} 3\times 10^{35}$ erg, and 10 CMEs predominantly from the closed-field regions (8/10) were confined. CMEs in closed-field regions exhibited lower kinetic energies, since part of the CME energy was expended to overcome magnetic tension and break open the overlying field. In conclusion our work suggests that eruptive CMEs in AB Dor may occur in high-latitude regions of open magnetic field, as the magnetic tension in such regions does not significantly inhibit the eruption.

• Evensberget, D; Marsden, S C; Carter, B D; Salmeron, R; Vidotto, A A & Folsom, C P [Show all 10 contributors for this article] (2023). The winds of young Solar-type stars in the Pleiades, AB Doradus, Columba, and β Pictoris. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 524(2), p. 2063–2063. doi: 10.1093/mnras/stad1650. Full text in Research Archive Show summary

  ABSTRACT Solar-type stars, which shed angular momentum via magnetized stellar winds, enter the main sequence with a wide range of rotational periods Prot. This initially wide range of rotational periods contracts and has mostly vanished by a stellar age $t\sim {0.6}\, {\rm Gyr}$, after which Solar-type stars spin according to the Skumanich relation $P_\text{rot}\propto \sqrt{t}$. Magnetohydrodynamic stellar wind models can improve our understanding of this convergence of rotation periods. We present wind models of 15 young Solar-type stars aged ∼24 Myr to ∼0.13 Gyr. With our previous wind models of stars aged ∼0.26 and ∼0.6 Gyr we obtain 30 consistent three-dimensional wind models of stars mapped with Zeeman–Doppler imaging – the largest such set to date. The models provide good cover of the pre-Skumanich phase of stellar spin-down in terms of rotation, magnetic field, and age. We find the mass-loss rate $\dot{M}\propto \Phi ^{{0.9\pm 0.1}}$ with a residual spread of ∼150 per cent and the wind angular momentum loss rate $\dot{J}\propto {}P_\text{rot}^{-1} \Phi ^{1.3\pm 0.2}$ with a residual spread of ∼500 per cent where Φ is the unsigned surface magnetic flux. When comparing different magnetic field scalings for each single star we find a gradual reduction in the power-law exponent with increasing magnetic field strength.

• Marsden, S C; Evensberget, D; Brown, E L; Neiner, C; Seach, J M & Morin, J [Show all 9 contributors for this article] (2023). The magnetic field and stellar wind of the mature late-F star χ Draconis A. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 522(1), p. 810–810. doi: 10.1093/mnras/stad925. Full text in Research Archive Show summary

  ABSTRACT F stars lie in the transition region between cool stars with dynamo generated fields and hot star fossil fields and offer an interesting window into the generation of magnetic fields in shallow convection zones. In this paper, we investigate the magnetic field of the mature F7V primary of the bright χ Draconis system. χ Dra was observed in circularly polarized light at four epochs from 2014 through to 2019 using the NARVAL spectropolarimeter at the Téléscope Bernard Lyot. Using the technique of least-squares deconvolution, we created high signal-to-noise line profiles from which we were able to measure the radial velocity of both the primary and secondary and use these to improve the orbital parameters of the system. Additionally, we used the technique of Zeeman Doppler imaging to reconstruct the large-scale magnetic field geometry of the primary at the four epochs. The magnetic maps show that χ Dra A has a predominately dipolar poloidal magnetic field at all epochs with the magnetic axis well aligned with the stellar rotation axis. The large-scale features of the magnetic field appear to be relatively stable over the ∼5 yr time base of our observations, with no evidence of any polarity reversals. We used the magnetic field maps to model the wind from χ Dra A at all epochs, showing that the mass-loss from the stellar wind of χ Dra A is 3–6 times the current solar value, while the angular momentum loss from the wind is around 3–4 times the solar value.

• Evensberget, D & Vidotto, A A (2023). Rotational evolution of young-to-old stars with data-driven three-dimensional wind models. Monthly Notices of the Royal Astronomical Society: Letters (MNRAS Letters). ISSN 1745-3925. 529(1). doi: 10.1093/mnrasl/slae010. Full text in Research Archive Show summary

  ABSTRACT Solar-type stars form with a wide range of rotation rates Ω. A wide Ω range persists until a stellar age of t ∼ 0.6 Gyr, after which solar-type stars exhibit Skumanich spin-down where Ω ∝ t−1/2. Rotational evolution models incorporating polytropic stellar winds struggle to simultaneously reproduce these two regimes, namely the initially wide Ω range and the Skumanich spin-down without imposing an a priori cap on the wind mass-loss rate. We show that a three-dimensional wind model driven by Alfvén waves and observational data yields wind torques that agree with the observed age distribution of Ω. In our models of the Sun and 27 open cluster stars aged from 0.04 to 0.6 Gyr that have observationally derived surface magnetic maps and rotation rates, we find evidence of exponential spin-down in young stars that are rapid rotators and Skumanich spin-down for slow rotators. The two spin-down regimes emerge naturally from our data-driven models. Our modelling suggests that the observed age distribution of stellar rotation rates Ω arises as a consequence of magnetic field strength saturation in rapid rotators.

• Evensberget, D; Carter, B D; Marsden, S C; Brookshaw, L; Folsom, C P & Salmeron, R (2021). The winds of young Solar-type stars in Coma Berenices and Hercules-Lyra. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 510(4), p. 5245–5245. doi: 10.1093/mnras/stab3557. Full text in Research Archive Show summary

  ABSTRACT We present wind models of 10 young Solar-type stars in the Hercules-Lyra association and the Coma Berenices cluster aged around ∼0.26 and ∼0.58 Gyr, respectively. Combined with five previously modelled stars in the Hyades cluster, aged ∼0.63 Gyr, we obtain a large atlas of 15 observationally based wind models. We find varied geometries, multi-armed structures in the equatorial plane, and a greater spread in quantities such as the angular momentum loss. In our models, we infer variation of a factor of ∼6 in wind angular momentum loss $\dot{J}$ and a factor of ∼2 in wind mass-loss $\dot{M}$ based on magnetic field geometry differences when adjusting for the unsigned surface magnetic flux. We observe a large variation factor of ∼4 in wind pressure for an Earth-like planet; we attribute this to variations in the ‘magnetic inclination’ of the magnetic dipole axis with respect to the stellar axis of rotation. Within our models, we observe a tight correlation between unsigned open magnetic flux and angular momentum loss. To account for possible underreporting of the observed magnetic field strength we investigate a second series of wind models where the magnetic field has been scaled by a factor of 5. This gives $\dot{M}\propto B^{0.4}$ and $\dot{J}\propto B^{1.0}$ as a result of pure magnetic scaling.

• Evensberget, D; Carter, B D; Marsden, S C; Brookshaw, L & Folsom, C P (2021). The winds of young Solar-type stars in the Hyades. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 506(2), p. 2335–2335. doi: 10.1093/mnras/stab1696. Full text in Research Archive Show summary

  ABSTRACT Stellar winds govern the spin-down of Solar-type stars as they age, and play an important role in determining planetary habitability, as powerful winds can lead to atmospheric erosion. We calculate 3D stellar wind models for five young Solar-type stars in the Hyades cluster, using TOUPIES survey stellar magnetograms and state-of-the-art Alfvén wave-driven wind modelling. The stars have the same 0.6 Gyr age and similar fundamental parameters, and we account for the uncertainty in and underestimation of absolute field strength inherent in Zeeman–Doppler imaging by adopting both unscaled and scaled (by a factor of five) field strengths. For the unscaled fields, the resulting stellar wind mass-loss is 2–4 times greater and the angular momentum loss 2–10 times greater than for the Sun today, with the scaled results correspondingly greater. We compare our results with a range published of wind models and for the Alfvén wave-driven modelling see evidence of mass-loss saturation at ${\sim 10} \dot{M}_{\odot }$.

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• Celledoni, Elena & Evensberget, Dag Frohde (2006). Numerical Simulation of Nonholonomic Dynamics. Norges teknisk-naturvitenskapelige universitet. Full text in Research Archive Show summary

  We study the numerical integration of nonholonomic problems. The problems are formulated using Lagrangian and Hamiltonian mechanics. We review briefly the theoretical concepts used in geometric mechanics. We reconstruct two nonholonomic variational integrators from the monograph of Monforte. We also construct two one-step integrators based on a combination of the continuous Legendre transform and the discrete Legendre transform from an article by Marsden and West. Inintially these integrators display promising behavior, but they turn out to be unstable. The variational integrators are compared with a classical Runge-Kutta method. We compare the methods on three nonholonomic systems: The nonholonomic particle from the monograph of Monforte, the nonholonomic system of particles from an article by McLachlan and Perlmutter, and a variation of the Chaplygin sleigh from Bloch.

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