A Disk-Wind Model for the Near-Infrared Excess Emission in Protostars

Abstract

Protostellar systems, ranging from low-luminosity classical T Tauri and Herbig Ae stars to highluminosity Herbig Be stars, exhibit a near-infrared (NIR) excess in their spectra that is dominated by a bump in the monochromatic luminosity with a peak near 3 µm. The bump can be approximated by a thermal emission component of temperature ∼ 1500 K that is of the order of the sublimation temperature of interstellar dust grains. In the currently popular "puffed-up rim" scenario, the bump represents stellar radiation that propagates through the optically thin inner region of the surrounding accretion disk and is absorbed and reemitted by the dust that resides just beyond the dust sublimation radius r sub . However, this model cannot account for the strongest bumps measured in these sources, and it predicts a pronounced secondary bounce in the interferometric visibility curve that is not observed. In this paper we present an alternative interpretation, which attributes the bump to reemission of stellar radiation by dust that is uplifted from the disk by a centrifugally driven wind. Winds of this type are a leading candidate for the origin of the strong outflows associated with protostars, and there is observational evidence for disk winds originating on scales ∼ r sub . Using a newly constructed Monte Carlo radiative transfer code and focusing on low-luminosity sources, we show that this model can account for the NIR excess emission even in bright Herbig Ae stars such as AB Auriga and MWC 275, and that it successfully reproduces the basic features of the visibilities measured in these protostars. We argue that a robust dusty outflow in these sources could be self-limitingthrough shielding of the stellar FUV photons -to a relatively narrow launching region between r sub and ∼ 2 r sub . We also suggest that the NIR and scattered-light variability exhibited by a source like MWC 275 can be attributed in this picture to the uplifting of dust clouds from the disk. Subject headings: circumstellar matter -ISM: jets and outflows -magnetohydrodynamics (MHD) -protoplanetary disks -radiative transfer -stars: individual (AB Auriga, MWC 275)

Resolving the inner disk of UX Orionis

“…The extended halo component contributes with 13 ± 2% (DDN model)/16 ± 4% (geometric model) to the total flux, similar as found in other young stellar objects with extended halo emission (∼9-16%, e.g. Monnier et al 2006;Chen et al 2012;Vural et al 2012Vural et al , 2014a that is interpreted as scattered emission from the outer disk surface or material outside the disk plane, for example, a dusty disk wind (e.g., Bans & Königl 2012).…”

The inner circumstellar disk of the UX Orionis star V1026 Scorpii

“…For this case, the derived inclination of V1026 Sco is still within the range predicted for UXOrs (45 • -68 • ) by Natta & Whitney (2000). Dust clouds in centrifugally-driven disk winds (Vinković & Jurkić 2007;Bans & Königl 2012) can also explain the UX Ori type variability of V1026 Sco, as they also are consistent with intermediate to high disk inclinations.…”

AA Tauri’s sudden and long-lasting deepening: enhanced extinction by its circumstellar disk

“…However, we note that these unstabilities may eventually trigger FU Ori outbursts as they propagate inward (Zhu et al 2010;Bae et al 2013), thus potentially making AA Tau a pre-FUOr candidate. Disk instabilities may also trigger dense disk winds (e.g., Lesur et al 2013;Bai & Stone 2013) that may be the source of enhanced lineof-sight extinction if they are able to lift up dust from the disk mid-plane (Owen et al 2011;Bans & Königl 2012). However, AA Tau's HeI 1083 nm line profile does not bear evidence for significant wind absorption.…”