Evidence for an Edge-On Disk around the Young Star MWC 778 from Infrared Imaging and Polarimetry

TL;DR

This study confirms the presence of a nearly-edge-on circumstellar disk around the young star MWC 778 using infrared imaging and polarimetry, revealing a complex envelope and spiral structures, and discusses the star's uncertain spectral classification.

Contribution

First direct imaging and polarimetry confirmation of an edge-on disk around MWC 778, along with detailed structure and envelope characterization.

Findings

Confirmed nearly-edge-on disk with inclination ~75 degrees

Detected spiral-shaped structure extending beyond 6 arcseconds

Identified discrepancies in spectral type and luminosity suggesting complex nature

Abstract

MWC 778 is an unusual and little-studied young stellar object located in the IC 2144 nebula. Recent spectroscopy by Herbig and Vacca (2008) suggested the presence of an edge-on circumstellar disk around it. We present near-infrared adaptive optics imaging polarimetry and mid-infrared imaging which directly confirm the suspected nearly-edge-on disk around MWC 778 (i ~ 70-80 degrees) plus reveal a more extensive envelope pierced by bipolar outflow cavities. In addition, our mid-infrared images and near-infrared polarization maps detect a spiral-shaped structure surrounding MWC 778, with arms that extend beyond 6" on either side of the star. Although MWC 778 has previously been classified as an Herbig Ae/Be star, the properties of its central source (including its spectral type) remain fairly uncertain. Herbig & Vacca (2008) suggested an F or G spectral type based on the presence of metallic absorption lines in the optical spectrum, which implies that MWC 778 may belong to the fairly rare class of Intermediate-Mass T Tauri Stars (IMTTSs) which are the evolutionary precursors to Herbig Ae/Be objects. Yet its integrated bolometric luminosity, > 750 L_sun (for an assumed distance of 1 kpc) is surprisingly high for an F or G spectral type, even for an IMTTS. We speculate on several possible explanations for this discrepancy, including its true distance being much closer than 1 kpc, the presence of a binary companion, and/or a non-stellar origin for the observed absorption lines.