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

Abstract

Context. It has been hypothesized that the location of Herbig Ae/Be stars (HAeBes) within the empirical relation between the inner disk radius (rin), inferred from K-band interferometry, and the stellar luminosity (L*), is related to the presence of the innermost gas, the disk-to-star accretion mechanism, the dust disk properties inferred from the spectral energy distributions (SEDs), or a combination of these effects. However, no general observational confirmation has been provided to date. Aims. This work aims to test whether the previously proposed hypotheses do, in fact, serve as a general explanation for the distribution of HAeBes in the size–luminosity diagram. Methods. GRAVITY/VLTI spectro-interferometric observations at ~2.2 μm have been obtained for five HBes representing two extreme cases concerning the presence of innermost gas and accretion modes. V590 Mon, PDS 281, and HD 94509 show no excess in the near-ultraviolet, Balmer region of the spectra (ΔDB), indicative of a negligible amount of inner gas and disk-to-star accretion, whereas DG Cir and HD 141926 show such strong ΔDB values that cannot be reproduced from magnetospheric accretion, but probably come from the alternative boundary layer mechanism. In turn, the sample includes three Group I and two Group II stars based on the Meeus et al. SED classification scheme. Additional data for these and all HAeBes resolved through K-band interferometry have been compiled from the literature and updated using Gaia EDR3 distances, almost doubling previous samples used to analyze the size–luminosity relation. Results. We find no general trend linking the presence of gas inside the dust destruction radius or the accretion mechanism with the location of HAeBes in the size–luminosity diagram. Similarly, our data do not support the more recent hypothesis linking such a location and the SED groups. Underlying trends are present and must be taken into account when interpreting the size–luminosity correlation. In particular, it cannot be statistically ruled out that this correlation is affected by dependencies of both L* and rin on the wide range of distances to the sources. Still, it is argued that the size–luminosity correlation is most likely to be physically relevant in spite of the previous statistical warning concerning dependencies on distance. Conclusions. Different observational approaches have been used to test the main scenarios proposed to explain the scatter of locations of HAeBes in the size–luminosity diagram. However, none of these scenarios have been confirmed as a fitting general explanation and this issue remains an open question.

Scanning disk rings and winds in CO at 0.01-10 au: a high-resolution $M$-band spectroscopy survey with IRTF-iSHELL

“…The dashed line shows the expected dust sublimation radius for a sublimation temperature of 1500 K as R subl = L /4πσT 4 subl , following previous work (Monnier & Millan-Gabet 2002;Dullemond & Monnier 2010;Lazareff et al 2017), which can partly provide an explanation but is unable to explain the data across the whole range in luminosities (see e.g. discussion in Marcos-Arenal et al 2021). As an estimate of R subl , we therefore take and outlier-resistant mean in six logarithmic bins, which is shown with an orange solid line in Figure 21, and interpolate that over the measured luminosities in our sample to obtain an estimate of R subl in each disk.…”

Identification and Spectroscopic Characterization of 128 New Herbig Stars*

“…In addition, there are alternative explanations to the change of trend in hydrogen line luminosity properties, apart from a change in accretion mode (see Mendigutía et al 2015;Marcos-Arenal et al 2021). For example, outflowing material could be dominating the line emission (caused by, e.g., disk photoevaporation, Guzmán-Díaz et al 2021).…”

Clustering Properties of Intermediate and High-mass Young Stellar Objects*

“…Between 1.5 < M < 8 M e , YSOs are categorized into the Herbig Ae/Be group (e.g., Vioque et al 2018), at the latest stages of pre-main-sequence evolution, and their cooler predecessors the intermediate-mass T Tauris (IMTTs; e.g., Valegård et al 2021). At M > 8 to 10 M e , YSOs are generally referred to as massive YSOs (MYSOs; e.g., Koumpia et al 2021;Marcos-Arenal et al 2021). In this work, we use the term "intermediateand highmass" when referring to any YSO with M > 1.5 M e and optically bright enough to be detected by Gaia (as opposed to the T Tauri regime containing optically bright low-mass YSOs).…”