Scattering and sublimation: a multi-scale view of $\mu$m-sized dust in the inclined disc of HD 145718

TL;DR

This study combines multi-instrument observations and modeling to analyze the dust properties, disc structure, and variability mechanisms of the inclined disc around HD 145718, revealing insights into grain sizes, disc geometry, and dust occultation.

Contribution

It provides a multi-scale analysis of dust scattering and sublimation in HD 145718's disc, linking observational data with radiative transfer models to characterize dust grain sizes and disc geometry.

Findings

Disc inclined at 67-71 degrees with a consistent position angle.

NIR scattering surface extends to ~75 au with specific aspect ratios.

Dust grains larger than λ/2π are present, supporting the disc surface layers.

Abstract

We present multi-instrument observations of the disc around the Herbig~Ae star, HD~145718, employing geometric and Monte Carlo radiative transfer models to explore the disc orientation, the vertical and radial extent of the near infrared (NIR) scattering surface, and the properties of the dust in the disc surface and sublimation rim. The disc appears inclined at $67-71^{\circ}$, with position angle, PA\,$=-1.0-0.6^{\circ}$, consistent with previous estimates. The NIR scattering surface extends out to $\sim75\,$au and we infer an aspect ratio, $h_{\rm{scat}}(r)/r\sim0.24$ in $J$-band; $\sim0.22$ in $H$-band. Our GPI images and VLTI+CHARA NIR interferometry suggest that the disc surface layers are populated by grains $\gtrsim \lambda/2\pi$ in size, indicating these grains are aerodynamically supported against settling and/or the density of smaller grains is relatively low. We demonstrate that our geometric analysis provides a reasonable assessment of the height of the NIR scattering surface at the outer edge of the disc and, if the inclination can be independently constrained, has the potential to probe the flaring exponent of the scattering surface in similarly inclined ($i\gtrsim70^{\circ}$) discs. In re-evaluating HD~145718's stellar properties, we found that the object's dimming events - previously characterised as UX~Or and dipper variability - are consistent with dust occultation by grains larger, on average, than found in the ISM. This occulting dust likely originates close to the inferred dust sublimation radius at $0.17\,$au.