Investigating the origin and spectroscopic variability of the near-infrared H i lines in the Herbig star VV Ser

Abstract

The origin of the near-infrared (NIR) H I emission lines in young stellar objects are not yet understood. To probe it, we present multi-epoch LBT-LUCIFER spectroscopic observations of the Paδ, Paβ, and Brγ lines observed in the Herbig star VV Ser, along with VLTI-AMBER Brγ spectro-interferometric observations at medium resolution. Our spectroscopic observations show line profile variability in all the H I lines. The strongest variability is observed in the redshifted part of the line profiles. The Brγ spectro-interferometric observations indicate that the Brγ line emitting region is smaller than the continuum emitting region. To interpret our results, we employed radiative transfer models with three different flow configurations: magnetospheric accretion, a magneto-centrifugally driven disc wind, and a schematic bipolar outflow. Our models suggest that the H I line emission in VV Ser is dominated by the contribution of an extended wind, perhaps a bipolar outflow. Although the exact physical process for producing such outflow is not known, this model is capable of reproducing the averaged single-peaked line profiles of the H I lines. Additionally, the observed visibilities, differential and closure phases are best reproduced when a wind is considered. Nevertheless, the complex line profiles and variability could be explained by changes in the relative contribution of the magnetosphere and/or winds to the line emission. This might indicate that the NIR H I lines are formed in a complex inner disc region where inflow and outflow components might coexist. Furthermore, the contribution of each of these mechanisms to the line appears time variable, suggesting a non-steady accretion/ejection flow.

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].…”

“…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].…”