Reading between the lines

Disk emission, wind, and accretion during the Z CMa NW outburst^★,★★

¹ SUPA, School of Science and Engineering, University of Dundee, Nethergate, Dundee DD1 4HN, UK
e-mail: a.siciliaaguilar@dundee.ac.uk
² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
⁴ IRAP Observatoire Midi-Pyrénées, 14 Avenue E Belin, 31400 Toulouse, France

Received: 25 May 2020
Accepted: 31 August 2020

Abstract

Aims. We use optical spectroscopy to investigate the disk, wind, and accretion during the 2008 Z CMa NW outburst.

Methods. Emission lines were used to constrain the locations, densities, and temperatures of the structures around the star.

Results. More than 1000 optical emission lines reveal accretion, a variable, multicomponent wind, and double-peaked lines of disk origin. The variable, non-axisymmetric, accretion-powered wind has slow (~0 km s⁻¹), intermediate (approximately −100 km s⁻¹), and fast (≥−400 km s⁻¹) components. The fast components are of stellar origin and disappear in quiescence, while the slow component is less variable and could be related to a disk wind. The changes in the optical depth of the lines between outburst and quiescence reveal that increased accretion is responsible for the observed outburst. We derive an accretion rate of 10⁻⁴ M_⊙ yr⁻¹ in outburst. The Fe I and weak Fe II lines arise from an irradiated, flared disk at ~0.5–3 × M_*/16 M_⊙ au with asymmetric upper layers, revealing that the energy from the accretion burst is deposited at scales below 0.5 au. Some line profiles have redshifted asymmetries, but the system is unlikely to be sustained by magnetospheric accretion, especially in outburst. The accretion-related structures extend over several stellar radii and, like the wind, are likely to be non-axisymmetric. The stellar mass may be ~6–8 M_⊙, lower than previously thought (~16 M_⊙).

Conclusions. Emission line analysis is found to be a powerful tool to study the innermost regions and accretion in stars within a very large range of effective temperatures. The density ranges in the disk and accretion structures are higher than in late-type stars, but the overall behavior, including the innermost disk emission and variable wind, is very similar for stars with different spectral types. Our work suggests a common outburst behavior for stars with spectral types ranging from M type to intermediate mass.