Spectroscopic analysis of accretion/ejection signatures in the Herbig Ae/Be stars HD 261941 and V590 Mon

Abstract

Herbig Ae/Be (HAeBe) stars are the intermediate-mass analogues of low-mass T Tauri stars. Both groups may present signs of accretion, outflow, and IR excess related to the presence of circumstellar discs. Magnetospheric accretion models are generally used to describe accreting T Tauri stars, which are known to have magnetic fields strong enough to truncate their inner discs and form accretion funnels. Since few HAeBe stars have had magnetic fields detected, they may accrete through a different mechanism. Our goal is to analyse the morphology and variability of emission lines that are formed in the circumstellar environment of HAeBe stars and use them as tools to understand the physics of the accretion/ejection processes in these systems. We analyse high-resolution (R ∼ 47 000) UVES/ESO spectra of two HAeBe stars – HD 261941 (HAe) and V590 Mon (HBe) that are members of the young (∼3 Myr) NGC 2264 stellar cluster and present indications of sufficient circumstellar material for accretion and ejection processes to occur. We determine stellar parameters with synthetic spectra, and also analyse and classify circumstellar lines such as H α, H β, and He i λ5875.7, according to their morphologies. We model the H α mean line profile, using a hybrid Magnetohydrodynamics (MHD) model that includes a stellar magnetosphere and a disc wind, and find signatures of magnetically driven outflow and accretion in HD 261941, while the H α line of V590Mon seems to originate predominantly in a disc wind.

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

“…Analogous spectro-interferometric studies devoted to fainter T Tauri stars are less frequent (see e.g., [75,76]), but in fact their emission line profiles are currently reproduced using hybrid models that consider magnetically driven accretion and winds (e.g., [33,34]). Similarly, the emission line profiles of HAeBes can also be reproduced either from MA modeling alone or combined with magnetically driven winds [56,61,74,[77][78][79][80][81]. Nonetheless, although many of these models are natural extensions from previous devoted to CTT stars, they lack a careful treatment of the larger projected rotational velocities that are characteristic of many HAeBes, which still constitutes a major limitation for the applicability of MA line modeling in these objects [28,74,81].…”

“…Similarly, the emission line profiles of HAeBes can also be reproduced either from MA modeling alone or combined with magnetically driven winds [56,61,74,[77][78][79][80][81]. Nonetheless, although many of these models are natural extensions from previous devoted to CTT stars, they lack a careful treatment of the larger projected rotational velocities that are characteristic of many HAeBes, which still constitutes a major limitation for the applicability of MA line modeling in these objects [28,74,81]. Other direct and indirect observational tests mentioned in the previous section that were important to support MA in CTTs can hardly be applied to HAeBes.…”

“…As noted in Section 3, MA line modeling including a complete treatment of the high rotational velocities that are typical in many HAeBes is still pending. Examples and details of line modeling applied to HAeBes, either considering MA alone or combined with magnetically driven winds, can be found in the literature [56,61,74,[77][78][79][80][81].…”

K-band GRAVITY/VLTI interferometry of “extreme” Herbig Be stars

“…Although V590 Mon has been considered an accreting source in the past, this could be associated with the significant variability of this star on timescales of decades (Pérez et al 2008;Moura et al 2020, and references therein). Indeed, the more recent work by Moura et al (2020) shows that the Hα emission currently shown by V590 Mon is associated to outflowing material in a disk wind and that accretion has a negligible contribution to its Hα line profile. A companion at > 5 ′′ from the central object and fainter by 6.6 magnitudes in the Kband was reported by Thomas et al (2007).…”

“…V590 Mon (aka LkHa 25, Walker 90) is a HBe star with a stellar temperature of ∼ 12500 K and mass of ∼ 3 M ⊙ (Fairlamb et al 2015;Moura et al 2020). A distance of 689 +57 −49 pc is inferred from the recent Gaia EDR3 parallax (Lindegren et al 2020).…”