High angular resolution near-ultraviolet polarization imaging of the Herbig Ae/Be star LK-H{\alpha}-233

TL;DR

This study uses high-resolution near-ultraviolet polarimetric imaging from Hubble to analyze the circumstellar environment of the Herbig Ae/Be star LK-Hα-233, revealing disk and outflow structures and their properties.

Contribution

It provides the first high-resolution polarimetric maps of LK-Hα-233 in the near-ultraviolet, uncovering detailed structures of its circumstellar disk and bipolar outflows.

Findings

Detection of a dark lane indicating a circumstellar disk or dust torus.

Identification of X-shaped bipolar outflows with centro-symmetric polarization patterns.

Estimation of outflow opening angles, system inclination, and star flux obscuration.

Abstract

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.Two of the key features of such stars are their circumstellar dusty disk and bipolar ionized outflows, which are key components for understanding planet formation processes and energy/matter deposition in the interstellar medium, respectively. In this context, imaging polarimetry is probably the sharpest tool to characterize 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. We take advantage of never published, near-ultraviolet polarimetric data of LK-H{\alpha}-233 taken by the Faint Object Camera aboard the Hubble Space Telescope in 1991, 1994 and 1995, which remained dormant in the archives despite their quality. Using the most recent and robust reduction pipeline for this instrument, we obtained high spatial resolution (0.0287 x 0.0287 arcsecond2) maps of this object at 4118 {\AA}, together with polarimetric measurements. A dark lane, bisecting the approaching and receding polar outflows, suggests the presence of a circumstellar disk or dust torus, obscuring the pre-main sequence star and collimating the ejecta. Polarization reveals that the outflows have an X-shape structure with a significant centro-symmetric pattern in polarization angle, indicating that the outflows are both hollow and scattering the emission from the buried star. We constrain the half-opening angle of both the outflows and circumstellar disk, determine the inclination of the system and estimate the obscured star's intrinsic flux. 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.