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

Abstract

Aims. We study the emission and absorption of CO ro-vibrational lines in the spectra of intermediate mass pre-main-sequence stars with the aim to determine both the spatial distribution of the CO gas and its physical properties. We also aim to correlate CO emission properties with disk geometry. Methods. Using high-resolution spectra containing fundamental and first overtone CO ro-vibrational emission, observed with CRIRES on the VLT, we probe the physical properties of the circumstellar gas by studying its kinematics and excitation conditions. Results. We detect and spectrally resolve CO fundamental ro-vibrational emission in 12 of the 13 stars observed, and in two cases in absorption.Conclusions. Keeping in mind that we studied a limited sample, we find that the physical properties and spatial distribution of the CO gas correlate with disk geometry. Flaring disks show highly excited CO fundamental emission up to v u = 5, while self-shadowed disks show CO emission that is not as highly excited. Rotational temperatures range between 250-2000 K. The 13 CO rotational temperatures are lower than those of 12 CO. The vibrational temperatures in self-shadowed disks are similar to or slightly below the rotational temperatures, suggesting that thermal excitation or IR pumping is important in these lines. In flaring disks the vibrational temperatures reach as high as 6000 K, suggesting fluorescent pumping. Using a simple kinematic model we show that the CO inner radius of the emitting region is ≈10 au for flaring disks and ≤1 au for self-shadowed disks. Comparison with hot dust and other gas tracers shows that CO emission from the disks around Herbig Ae/Be stars, in contrast to T Tauri stars, does not necessarily trace the circumstellar disk up to, or inside the dust sublimation radius, R subl . Rather, the onset of the CO emission starts from ≈R subl for self-shadowed disks, to tens of R subl for flaring disks. It has recently been postulated that group I Herbig stars may be transitional disks and have gaps. Our CO observations are qualitatively in agreement with this picture. We identify the location of the CO emission in these group I disks with the inner rim of the outer disk after such a gap, and suggest that the presence of highly vibrationally excited CO emission and a mismatch between the rotational and vibrational temperature may be a proxy for the presence of moderately sized disk gaps in Herbig Ae/Be disks.

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…Most spectra have been taken as part of the ESO Large Program 179.C-0151 (Pontoppidan et al 2011b); some measurements have been added from programs 079.C-0349 and 081.C-0833 as published in van der Plas et al (2015), from program 091.C-0671 as published in Carmona et al (2016), and from program 088. C-0898 as published in Hein Bertelsen et al (2016).…”

“…C-0898 as published in Hein Bertelsen et al (2016). CRIRES has a resolving power of ∼96,000 or 3.2 km s −1 , which provides detailed velocity-resolved line profiles of CO gas from inner disks and allows us to radially locate the emission through Keplerian line broadening (e.g., Brown et al 2013;van der Plas et al 2015). In total, we include in this work 52 disks that have velocityresolved CO lines from CRIRES observations.…”

“…Stellar masses, luminosities, distances, accretion rates and luminosities, are taken from Salyk et al (2013) and references therein for solar-mass stars and from Fairlamb et al (2015) for intermediate-mass stars, unless noted otherwise. Disk inclinations are taken from spatially resolved dust observations or velocity-resolved CO observations as compiled in and in van der Plas et al (2015). Values for v v 2 1 and R co are measured as explained in and Equation (1).…”

“…Line fluxes and errors are reported in Table 6; in all figures throughout this , where σ is the sum of pixel flux errors. For targets marked with * , the CO line flux is adopted from van der Plas et al (2015), Hein Bertelsen et al (2016, and Brittain et al (2007), while the H 2 O and OH line fluxes at 2.9 μm, where available, are adopted from Fedele et al (2011). paper, line fluxes are normalized to a common distance of 140 pc for a homogeneous comparison, using distances from Table 2.…”

“…Gas spectroscopy holds a unique potential to unveil the chemical evolution of planetforming regions. Recent developments in sensitivity and spectral resolution of infrared spectrographs allowed the first large surveys of molecular gas in inner disks able to spectrally (and sometimes also spatially) resolve gas within dust gaps (e.g., Pontoppidan et al 2008;Salyk et al 2011a;van der Plas et al 2015). The combined analysis of a large sample of disks recently established rovibrational CO (carbon monoxide) emission at 4.7 μm as a tracer of inner gaps in molecular gas.…”