Warm millimetre dust in protoplanetary discs near massive stars

Thomas J Haworth

doi:10.1093/mnras/stab728

Abstract

ABSTRACT Dust plays a key role in the formation of planets and its emission also provides one of our most accessible views of protoplanetary discs. If set by radiative equilibrium with the central star, the temperature of dust in the disc plateaus at around 10–20 K in the outer regions. However, sufficiently nearby massive stars can heat the outer disc to substantially higher temperatures. In this paper, we study the radiative equilibrium temperature of discs in the presence of massive external sources and gauge the effect that it has on millimetre dust mass estimates. Since millimetre grains are not entrained in any wind, we focus on geometrically simple 2D axisymmetric disc models using radiative transfer calculations with both the host star and an external source. Recent surveys have searched for evidence of massive stars influencing disc evolution using disc properties as a function of projected separation. In assuming a disc temperature of 20 K for a disc a distance D from a strong radiation source, disc masses are overestimated by a factor that scales with D−1/2 interior to the separation that external heating becomes important. This could significantly alter dust mass estimates of discs in close proximity to θ1C in the Orion Nebular Cluster (ONC). We also make an initial assessment of the effect upon snow lines. Within a parsec of an O star like θ1C a CO snow line no longer exists, though the water snow line is virtually unaffected except for very close separations of $\le 0.01\,$pc.

Highlights

We have known about the impact of environment on circumstellar discs essentially for as long as we have been able to directly image them (O’dell & Wen 1994), there has recently been a resurgence of interest in the topic.Most stars form in clustered environments (e.g. Lada & Lada 2003; Krumholz et al 2019)
Given that 3D models would be required for arbitrary external radiation orientations we argue that this situation is the prudent choice
We study the deviation from the simple analytic solution in our other models in section 4.1, finding that the analytic approximation is typically good in the regions of the disc where the external field sets the temperature, but typically overestimates the disc temperature by a factor of around 4 where the host star dominates

Summary

Introduction

We have known about the impact of environment on circumstellar discs essentially for as long as we have been able to directly image them (O’dell & Wen 1994), there has recently been a resurgence of interest in the topic.Most stars form in clustered environments (e.g. Lada & Lada 2003; Krumholz et al 2019). The three main ways that a cluster environment affects discs is through external photoevaporation, dynamical (gravitational) encounters and through compositional inheritence and enrichment (e.g. of short lived radionuclides like Aluminium 26, Lichtenberg et al 2019; Reiter 2020, though we do not focus on this here). The unprecedented sensitivity and resolution that ALMA provides allows us to analyse disc dust mass and radius statistics throughout star forming regions (e.g. Mann et al 2014; Ansdell et al 2017; Eisner et al 2018; Boyden & Eisner 2020; Ansdell et al 2020). Trends in disc properties as a function of projected separation from the strongest UV sources is often interpreted as evidence for external photoevaporation (Mann et al 2014; Ansdell et al 2017; Eisner et al 2018).

Objectives

Results

Conclusion