Magnetic geometry and surface differential rotation of the bright Am star Alhena A

Abstract

Alhena A (γ Gem A) is a bright Am star, with the strongest disc-integrated magnetic field strength reported so far for an Am star. Its spectrum exhibits standard circularly polarized Zeeman signatures, contrary to all previously studied Am stars that display abnormal signatures dominated by a single-signed lobe.We present here the result of follow-up observations of Alhena, using very high signal-to-noise spectropolarimetric data obtained over 25 observing nights with NAR-VAL at Télescope Bernard Lyot, in the frame of the BRITE (BRIght Target Explorer) spectropolarimetric survey.We confirm that Alhena A is magnetic and we determine its surface magnetic properties using different methods. Inclined dipole models are used to reproduce the longitudinal field measurements, as well as the Stokes V line profiles themselves. In both cases, the model is consistent with a polar field strength of ∼ 30 G. This is confirmed by a Zeeman-Doppler Imaging (ZDI) model, which also unveils smaller scale magnetic structures. A rotational period of 8.975 days was identified using intensity line profile variations. The ZDI inversion suggests that the surface magnetic field is sheared by differential rotation, with a difference in rotation rate between high and low latitudes at about 15% of the solar value. This result challenges theories of the development of surface differential rotation in intermediate mass main sequence stars.

MOBSTER -- VI. The crucial influence of rotation on the radio magnetospheres of hot stars

“…Using the fundamental parameters (equatorial radius Req = 2.008 ± 0.006 R⊙, M * = 1.86 ± 0.03 M⊙) and rotation period Prot = 0.323 d determined via careful interferometric modelling performed by Bouchaud et al (2020) yields a critical rotation parameter W = 0.75 and a Kepler corotation radius RK = 1.2 R * . The star's CAK mass-loss rate is Ṁ = 10 −13 M⊙ yr −1 ; assuming a terminal velocity of 3000 km s −1 , RK will be inside the Alfvén surface so long as B d > 0.1 G, well within the upper limits on Altair's surface magnetic field and consistent with the range of ultra-weak fields detected in other main sequence A-type stars (Petit et al 2010a;Blazère et al 2020).…”

“…The expected radio luminosity from the breakout scaling is then log L rad /L⊙ = −12, translating at 1 cm to 0.15 µJy at Vega's 7.67 pc distance: certainly undetectable, since this is much less than the expected 1 cm photospheric flux of about 0.5 mJy. Radio observations of other stars with ultra-weak fields do not seem to be available, although at least in the case of Alhena the relatively long ∼ 9 d period and ∼30 G surface field makes it unlikely the star would produce detectable emission (Blazère et al 2016a(Blazère et al , 2020, while in the cases of β UMa and θ Leo (Blazère et al 2016b) the rotational periods are not known, making their radio luminosities impossible to estimate.…”

Testing the fossil field hypothesis: could strongly magnetized OB stars produce all known magnetars?

“…Are they magnetic? Based on recent measurements the weak (1-10 G) magnetic field of A-F star such as Vega (A0 V, Ligniéres et al 2009), β UMa (A1 IV, Blazère et al 2016b) and others (Neiner et al 2017c;Blazère et al 2020) one can suppose that all these stars have the root-mean square (rms) magnetic field in the interval [0.1 -10] G and may be called weakly-magnetic stars (also known as "ultraweak field stars" or "ultra-weakly magnetic stars"). Moreover, there is already a series of works that prove in different ways that magnetic massive stars have bimodal distribution of magnetic field based on observations (Aurière et al 2007;Grunhut et al 2017) and based on theoretical or numerical techniques (Cantiello & Braithwaite 2019;Jermyn & Cantiello 2020).…”

KELT-17: a chemically peculiar Am star and a hot-Jupiter planet

“…Chemically peculiar Am stars rotate more slowly than average A-type stars (e.g., Abt 2000;Niemczura et al 2015), and most of them belong to binary systems (e.g., North et al 1998;Carquillat & Prieur et al 2007;Smalley et al 2014). These stars may host weak or ultraweak magnetic fields driven by surface convection (e.g., Folsom et al 2013;Blazère et al 2016Blazère et al , 2020. The origin of their peculiar abundances is commonly attributed to diffusion processes due to gravitational settling and radiative levitation (Michaud 1970;Michaud et al 1976Michaud et al , 1983Vauclair et al 1978;Alecian Based on data obtained at Complejo Astronómico El Leoncito, operated under an agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.…”