Solar and Stellar Astrophysics (astro-ph.SR)

Abstract: Two recently discovered white dwarfs, WDJ041246.84$+$754942.26 and WDJ165335.21$-$100116.33, exhibit H$\alpha$ and H$\beta$ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as $B < 0.05$ MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WDJ041246.84$+$754942.26 the strength of the H$\alpha$ and H$\beta$ emission cores varies in anti-phase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung-Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars.

Abstract: IW Per, a single-lined spectroscopic binary with a short period of 0.92d, is known to be a A-type metallic-line (Am) star showing anomalous line strengths of specific elements. Previously, Kim (1980) reported that its equivalent widths of CaII 3934, SrII 4215, and ScII 4320 lines (important key lines characterizing the Am anomaly) show cyclic variations in accordance with the rotation phase, implyig that the chemical peculiarities on the surface are not uniform but of rather patchy distribution, though no trial of reconfirmation seems to have been done so far. In order to check the validity of this finding, 10 high-dispesion spectra of IW Per covering different phases were analyzed for these lines by using the spectrum-fitting technique to determine the abundances of Ca, Sr, and Sc and the corresponding equivalent widths. It turned out, however, that no firm evidence of such phase-dependent line-strength variations could be found, suggesting that significant chemical inhomogeneity on the surface of IW Per is unlikely to exist, at least as regards to the period of our observations (2010 December). Meanwhile, the abundances of O, Si, Ca, Ba, and Fe resulting from the 6130-6180A region corroborate that IW Per is a distinct Am star despite that its rotational velocity (~100 km/s) is near to the existent limit of Am phenomenon.

Abstract: The lowest-order force balance in rotating stars is between gravity, pressure, and the centrifugal force (here referred to as 'GPR' balance). GPR balance determines both the stellar oblateness and the aspherical thermal anomalies. Here we emphasize a subtle point. Stellar thermal wind balance is simply the curl of GPR balance and the stellar thermal wind should be regarded as the baroclinic component of the oblateness. The thermal wind thus determines only part of the aspherical thermal anomalies, which have both baroclinic and barotropic contributions. Here we treat the full oblateness, including the thermal wind, using pressure coordinates. We derive the generalised stellar thermal wind equation and identify the parameter regime for which it holds. In the case of the Sun, not including the oblateness has resulted in conflicting calculations of the theoretical aspherical temperature anomaly. We provide new calculation here and find that the baroclinic anomaly from the thermal wind is ~3-60 times smaller than the barotropic anomaly and may not be measurable helioseismically. If measurement were possible, this would potentially yield a new way to bracket the depth of the solar tachocline.