High angular resolution near-ultraviolet polarization imaging of the Herbig Ae/Be star LK-Hα-233

Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France

^⋆ Corresponding author: frederic.marin@astro.unistra.fr

Received: 24 March 2025
Accepted: 20 May 2025

Abstract

Context. Herbig Ae/Be stars are young, pre-main-sequence stars that provide critical insights into the processes of stellar formation, early stellar evolution, and protoplanetary disks.

Aims. Two key features of such stars are their circumstellar dusty disk and bipolar ionized outflows, which are critical components for understanding planet formation processes and the deposition of energy and/or matter in the interstellar medium, respectively. In this context, imaging polarimetry is arguably the most precise tool for characterizing the various structures and dynamics around the central star, due to the sensitivity of polarization to the morphology of the emitting, scattering, and absorbing media.

Methods. We used previously unpublished, near-ultraviolet polarimetric data of LK-Hα-233 taken by the Faint Object Camera aboard the Hubble Space Telescope in 1991, 1994 and 1995, which remained archived despite their quality. Using the most recent and robust reduction pipeline for this instrument, we obtained high spatial resolution (0.0287 × 0.0287 arcsecond²) maps of this object at 4118 Å, together with polarimetric measurements.

Results. A dark lane, bisecting the approaching and receding polar outflows, suggests the presence of a circumstellar disk or dust torus, which obscures the pre-main-sequence star and collimates the ejecta. Polarization reveals that the outflows have an X-shaped structure with a significant centrosymmetric pattern in the polarization angle, indicating that the outflows are both hollow and scatter the emission from the buried star. We constrain the half-opening angle of both the outflows and the circumstellar disk, determine the inclination of the system, and estimate the obscured star’s intrinsic flux.

Conclusions. This study highlights the importance of high-resolution polarimetric observations in understanding the complex environment around Herbig Ae/Be stars and advocates for future similar instruments.