Understanding discs in binary YSOs – detailed modelling of VV CrA

Abstract

Given that a majority of stars form in multiple systems, in order to fully understand the starand planet-formation processes we must seek to understand them in multiple stellar systems. With this in mind, we present an analysis of the enigmatic binary T-Tauri system VV Corona Australis, in which both components host discs, but only one is visible at optical wavelengths. We seek to understand the peculiarities of this system by searching for a model for the binary which explains all the available continuum observations of the system. We present new midinfrared interferometry and near-infrared spectroscopy along with archival millimetre-wave observations, which resolve the binary at 1.3 mm for the first time. We compute a grid of pre-main-sequence radiative transfer models and calculate their posterior probabilities given the observed spectral energy distributions and mid-infrared interferometric visibilities of the binary components, beginning with the assumption that the only differences between the two components are their inclination and position angles. Our best-fitting solution corresponds to a relatively low luminosity T-tauri binary, with each component's disc having a large scale height and viewed at moderate inclination (∼ 50• ), with the infrared companion inclined by ∼ 5• degrees more than the primary. Comparing the results of our model to evolutionary models suggests stellar masses ∼ 1.7 M and an age for the system of 3.5 Myr, towards the upper end of previous estimates. Combining these results with accretion indicators from near-IR spectroscopy, we determine an accretion rate of 4.0 ×10 −8 M yr −1 for the primary. We suggest that future observations of VV CrA and similar systems should prioritise high angular resolution sub-mm and near-IR imaging of the discs and high resolution optical/NIR spectroscopy of the central stars.

S and VV Corona Australis: Spectroscopic Variability in Two Young Binary Star Systems

“…We find the mass accretion rate for S CrA A to be ∼ 9 × 10 −8 M /yr, the mass accretion rate for S CrA B and VV CrA A to be ∼ 3 × 10 −8 M /yr, and that of VV CrA B to be ∼ 8 × 10 −8 M /yr. The derived accretion rate for VV CrA A agrees with the mass accretion rate estimate from Scicluna et al (2016) of ∼ 4 × 10 −8 M /yr, but we find that the accretion rates for S CrA A and B are lower than those measured in 2018 by Gahm et al (2018), who found that the accretion rate was ∼ 5 × 10 −7 M /yr, about an order of magnitude higher than we measure for both stars.…”

“…We used unresolved JHKL photometry (Table 1) with our flux ratios (Table 3) to derive NIR stellar fluxes for individual components of S CrA and VV CrA. We show the component-resolved K-band (2.2µm) through Q-band (∼ 18µm) photometry of McCabe et al (2006) and Scicluna et al (2016) for S CrA and VV CrA, respectively, in Table 8. Our SEDs are shown in Figure 5.…”

“…S CrA and VV CrA are young enough that their photospheres remain heavily obscured (A v ∼ 2 for both components of S CrA and VV CrA A, and A v = 10.2 for VV CrA B, Varga et al 2018), although the systems themselves are bright in the NIR, the result of excess emission from their warm, optically thick circumstellar disks and high accretion rates. Both S CrA and VV CrA exhibit variable optical and NIR veiling over time (Bellingham & Rossano 1980;Eiroa et al 2002;Walter & Miner 2005;Köhler et al 2008;Kóspál et al 2012;Vural et al 2012;Scicluna et al 2016), indicating that they are both experiencing variable accretion. S CrA is one of the earliest stars identified as a T Tauri variable (Joy 1945) and was identified as an ≈ 1" binary in the same paper.…”

“…VV CrA has primarily been studied in two capacities: first, as an IRC host system (Koresko et al 1997), and second, with spectroscopy to determine disk composition (e.g., Yudin 2000;Cieza et al 2009;Kruger et al 2011;Lindberg et al 2015). Scicluna et al (2016) produced detailed modeling of both components of VV CrA and their disks based on data taken at 1.3 mm, in the mid infrared, and in the K-band. They were not able to determine the spectral types of the stars in the system because strong veiling obscured the photospheric absorption lines at the time of their K-band observations.…”

VLTI/MIDI atlas of disks around low- and intermediate-mass young stellar objects

“…Young multiple stellar systems were also valuable targets for MIDI observations. By determining disk geometries and orientations, cloud fragmentation and binary formation theories can be tested (Ratzka et al 2009;Roccatagliata et al 2011;Scicluna et al 2016). Tidal truncation of disks was observed in the hierarchical triple system T Tau by Ratzka (2008).…”