Revealing the sub-AU asymmetries of the inner dust rim in the disk around the Herbig Ae star R Coronae Austrinae

Abstract

Context. Unveiling the structure of the disks around intermediate-mass pre-main-sequence stars (Herbig Ae/Be stars) is essential for our understanding of the star and planet formation process. In particular, models predict that in the innermost AU around the star, the dust disk forms a "puffed-up" inner rim, which should result in a strongly asymmetric brightness distribution for disks seen under intermediate inclination.Aims. Our aim is to constrain the sub-AU geometry of the inner disk around the Herbig Ae star R CrA and search for the predicted asymmetries. Methods. Using the VLTI/AMBER long-baseline interferometer, we obtained 24 near-infrared (H-and K-band) spectrointerferometric observations on R CrA. Observing with three telescopes in a linear array configuration, each data set samples three equally spaced points in the visibility function, providing direct information about the radial intensity profile. In addition, the observations cover a wide position angle range (∼97 • ), also probing the position angle dependence of the source brightness distribution. Results. In the derived visibility function, we detect the signatures of an extended (Gaussian FWHM ∼ 25 mas) and a compact component (Gaussian FWHM ∼ 5.8 mas), with the compact component contributing about two-thirds of the total flux (both in H-and K-band). The brightness distribution is highly asymmetric, as indicated by the strong closure phases (up to ∼40 • ) and the detected position angle dependence of the visibilities and closure phases. To interpret these asymmetries, we employ various geometric as well as physical models, including a binary model, a skewed ring model, and a puffed-up inner rim model with a vertical or curved rim shape. For the binary and vertical rim model, no acceptable fits could be obtained. On the other hand, the skewed ring model and the curved puffed-up inner rim model allow us to simultaneously reproduce the measured visibilities and closure phases. From these models we derive the location of the dust sublimation radius (∼0.4 AU), the disk inclination angle (∼35 • ), and a north-south disk orientation (PA ∼ 180−190 • ). Our curved puffed-up rim model can reproduce reasonably well the interferometric observables and the SED and suggests a luminosity of ∼29 L and the presence of relatively large ( > ∼ 1.2 μm) Silicate dust grains. Our study also reveals discrepancies between the measured interferometric observables and the puffed-up inner rim models, providing important constraints for future refinements of these theoretical models. Perpendicular to the disk, two bow shock-like structures appear in the associated reflection nebula NGC 6729, suggesting that the detected sub-AU size disk is the driving engine of a large-scale outflow. Conclusions. Detecting, for the first time, strong non-localized asymmetries in the inner regions of a Herbig Ae disk, our study supports the existence of a puffed-up inner rim in YSO disks.

The 2008 outburst in the young stellar system Z CMa

“…Considering a dust sublimation temperature around 1500-2000 K (Pollack et al 1994), and the stellar properties determined by van den Ancker et al (2004), the inner edge of the dusty disk must be located at ∼4-7 AU, in agreement with our findings. An asymmetry in the inclined inner disk could explain the non-zero closure phases measured at a level similar to other Herbig AeBe stars (Kraus et al 2009;Benisty et al 2010). …”

The structure of disks around Herbig Ae/Be stars as traced by CO ro-vibrational emission

“…(a) Lagage et al (2006); (b) Marraco & Rydgren (1981); (c) Acke & van den Ancker (2006); (d) Ardila et al (2007); (e) Pontoppidan et al (2008); ( f ) de Zeeuw et al (1999); (g) Acke & van den Ancker (2004); (h) Meeus et al (1998); (i) van der Plas et al (2008); (k) Grady et al (2004); (l) Catala et al (2007); (m) Eisner et al (2004); (n) Verhoeff et al (2010); (o) van Boekel et al (2005); (q) Kraus et al (2009); (r) Forbrich et al (2006); (s) Merín et al (2004); (t) Weinberger et al (1999); (u) Fedele et al (2008); (v) Fukagawa et al (2003); (w) Kraus et al (2008); (x) Blondel & Tjin A Djie (2006); (y) van den Ancker et al (1998).…”

The interferometric view of Herbig Ae/Be stars

“…Observations with the VLTI/AMBER and Keck/V2-SPR instrument support this interpretation for T Tauri and very late-type Herbig Ae stars, where the visibilities measured within the line indicate that the line emission originates from just a few stellar radii from the star (Kraus et al 2009a;Eisner et al 2010Eisner et al , 2014. However, many high-luminosity sources show a significantly more extended line-emission region, which is not consistent with a magnetospheric accretion scenario.…”

“…More detailed studies with longer baselines were conducted on individual objects. For instance, Kraus et al (2009a) measured significant non-zero closure phase signals on R Coronae Australis. The visibility and closure phase signals also varied systematically with baseline position angle, which could be modelled with a curved rim model that takes the gas pressure-dependence of the dust sublimation temperature into account (Isella and Natta 2005).…”

“…This extended emission can contribute up to ∼ 30% of the total H and K-band flux in some systems (e.g. Kraus et al 2009a) and is typically distributed over a few hundred milliarcseconds to a few arcseconds (Leinert et al 2001). The physical origin of the emission is still debated, but it might correspond to scattered light from the outer disk (Pinte et al 2008), from an halo in which the disk is embedded, or the walls of an outflow cone.…”