Medium-resolution échelle spectroscopy of the Red Square Nebula, MWC 922

Abstract

Context. Medium-resolution échelle spectra of the Red Square Nebula surrounding the star MWC 922 are presented. The spectra have been obtained in 2010 and 2012 using the X-shooter spectrograph mounted on the Very Large Telescope (VLT) in Paranal, Chile. The spectrum covers a wavelength range between 300 nm−2.5 µm and shows that the nebula is rich in emission lines. Aims. We aim to identify the emission lines and use them as a tool to determine the physical and chemical characteristics of the nebula. The emission lines are also used to put constraints on the structure of the nebula and on the nature of the central stars. Results. These findings put constraints on the evolution of the central objects in MWC 922. The Red Square shows strong similarities to the Red Rectangle Nebula, both in morphology and in its mid-IR spectroscopic characteristics. As for the Red Rectangle, the observed morphology of the nebula reflects mass-loss in a binary system. Specifically, we attribute the biconical morphology and the associated rung-like structure to the action of intermittent jets blown by the accreting companion in a dense shell, which has been created by the primary. We stress, though, that despite the morphological similarities, these two objects represent very different classes of stellar objects.

A rotating fast bipolar wind and disk system around the B[e]-type star MWC 922

“…Adopting an average H 2 density of ≈10 4 -10 5 cm −3 in the hot CO emitting region and using the column density estimated by Wehres et al (2017) we deduce that the size of the intervening CO-emitting region along the line of sight is probably Δ r ≈1-10 au. Note that the density of the hot CO-emitting gas is plausibly intermediate between that in the central H ii region studied here (~10 6 -10 7 cm −3 , § 4) and that of the more extended surrounding nebulosity traced by numerous forbidden and permitted emission lines in the optical (in the range ≈10-10 4 cm −3 , see Wehres et al 2017; Bally & Chia 2019). If we assume that the hot CO gas is located in a hollow cylindrical structure with a representative inner radius of R , and a thickness (Δ r ) and a scale height h ~Δ r , then we deduce a total mass in this component of only MH2hot≈[10−7‐10−8]×R1000auMfalse⊙.…”

“…Part of this neutral gas component is indeed detected by means of first and second overtone CO transitions in the near IR (Wehres et al 2017). Wehres et al (2017) estimate a gas temperature of about 3000 K in the CO emitting region and conclude that this molecule lies in a disk/ring-like structure, surviving dissociation due to dust in the inner disk blocking the stellar UV radiation by the B[e] central star. These authors deduce a relatively low CO column density of ~1.7×10 15 cm −2 , which implies a H 2 column density of ~8.5×10 18 cm −2 adopting a typical CO-to-H 2 fractional abundance of X(CO)=2×10 −4 .…”

“…These authors deduce a relatively low CO column density of ~1.7×10 15 cm −2 , which implies a H 2 column density of ~8.5×10 18 cm −2 adopting a typical CO-to-H 2 fractional abundance of X(CO)=2×10 −4 . Wehres et al (2017) do not attempt to compute the total mass of hot molecule rich gas traced by NIR CO emission false(MH2hotfalse) probably because the dimensions of the CO emitting volume are unknown. In the following paragraph, we provide a crude estimation of the mass that suggests that, as MHII,MH2hot probably represents only a very small fraction of the mass traced by the mm-continuum dust emission.…”

“…The radius of the hot CO-ring is unknown since none of the slits used by Wehres et al (2017) were oriented along the nebula equator. However, we believe that values much larger than ≈1000 au are unlikely since it would imply that the hot CO emission is reaching to, and coexisting with, the less dense and relatively extended optical/NIR nebulosity where the gas is known to be atomic and begins to be dissociated/ionized by the external (ISM) UV radiation field (e.g.…”

“…Cohen et al 1975; Jura et al 1995; Waters et al 1998; Bujarrabal et al 2016). For a comprehensive description of MWC 922’s characteristics, see, for example, CSC+17 and references therein, or the most recent studies by Wehres et al (2017) and Bally & Chia (2019). These works report on 300 nm-2.5 μ m spectroscopy of the rich emission line spectrum in this target, including the detection of PAH and CO first and second overtone emission (Wehres et al 2017), and deep narrow-band optical imaging showing the presence of a ~[0.8-10]×10 4 years old, pc-scale jet perpendicular to the disk and expanding at ~500 km s −1 (Bally & Chia 2019).…”