The accretion rates and mechanisms of Herbig Ae/Be stars

This work presents a spectroscopic study of 163 Herbig Ae/Be stars. Amongst these, we present new data for 30 objects. Stellar parameters such as temperature, reddening, mass, luminosity, and age are homogeneously determined. Mass accretion rates are determined from Hα emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gaia DR2 parallaxes giving a total of 78 objects with homogeneously determined stellar parameters and mass accretion rates. In addition, mass accretion rates of an additional 85 HAeBes are determined. We confirm previous findings that the mass accretion rate increases as a function of stellar mass, and the existence of a different slope for lower and higher mass stars, respectively. The mass where the slope changes is determined tobe 3.98+1.37−0.94M⊙⁠. We discuss this break in the context of different modes of disc accretion for low- and high-mass stars. Because of their similarities with T Tauri stars, we identify the accretion mechanism for the late-type Herbig stars with the Magnetospheric Accretion. The possibilities for the earlier-type stars are still open, we suggest the Boundary Layer accretion model may be a viable alternative. Finally, we investigated themass accretion–age relationship. Even using the superior Gaia based data, it proved hard to select a large enough sub-sample to remove the mass dependence in this relationship. Yet, it would appear that the mass accretion does decline with age as expected from basic theoretical considerations

Optical and near-infrared observations of the Fried Egg Nebula

Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate and geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to provide insight into the nature (i.e. geometry, rates) of mass-loss episodes, and in particular, the connection between the observed asymmetries due to the mass lost in a fast wind or during a previous, prodigious mass-losing phase. In this context, yellow hypergiants offer a good opportunity to study mass-loss events. Methods. We analysed a large set of optical and near-infrared data in spectroscopic and photometric, spectropolarimetric, and interferometric (GRAVITY/VLTI) modes, towards the yellow hypergiant IRAS 17163−3907. We used X-shooter optical observations to determine the spectral type of this yellow hypergiant and we present the first model-independent, reconstructed images of IRAS 17163−3907 at these wavelengths tracing milli-arcsecond scales. Lastly, we applied a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published diffraction-limited VISIR images at 8.59 μm, 11.85 μm, and 12.81 μm simultaneously, adopting a revised distance determination using Gaia Data Release 2 measurements. Results. We constrain the spectral type of IRAS 17163−3907 to be slightly earlier than A6Ia (Teff ∼ 8500 K). The interferometric observables around the 2 μm window towards IRAS 17163−3907 show that the Brγ emission appears to be more extended and asymmetric than the Na I and the continuum emission. Interestingly, the spectrum of IRAS 17163−3907 around 2 μm shows Mg II emission that is not previously seen in other objects of its class. In addition, Brγ shows variability in a time interval of four months that is not seen towards Na I. Lastly, in addition to the two known shells surrounding IRAS 17163−3907, we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The 2 μm continuum originates directly from the star and not from hot dust surrounding the stellar object. The observed spectroscopic variability of Brγ could be a result of variability in the mass-loss rate. The interpretation of the presence of Na I emission at closer distances to the star compared to Brγ has been a challenge in various studies. To address this, we examine several scenarios. We argue that the presence of a pseudo-photosphere, which was traditionally considered to be the prominent explanation, is not needed and that it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform theories of mass-loss mechanisms, which is a topic still under debate both in theory and observations. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms

K-band GRAVITY/VLTI interferometry of "extreme" Herbig Be stars. The size-luminosity relation revisited.

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

Reading between the lines:Disk emission, wind, and accretion during the Z CMa NW outburst

(Abridged) We use optical spectroscopy to investigate the disk, wind, and accretion during the 2008 ZCMa NW outburst. Over 1000 optical emission lines reveal accretion, a variable, multi-component wind, and double-peaked lines of disk origin. The variable, non-axisymmetric, accretion-powered wind has slow ($\sim$0 km s$^{-1}$), intermediate ($\sim -$100 km s$^{-1}$) and fast ($\geq -$400 km s$^{-1}$) components. The fast components are of stellar origin and disappear in quiescence, while the slow component is less variable and could be related to a disk wind. The changes in the optical depth of the lines between outburst and quiescence are consistent with increased accretion being responsible for the observed outburst. We derive an accretion rate of 10$^{-4}$ M$_\odot$/yr in outburst. The Fe I and weak Fe II lines arise from an irradiated, flared disk at $\sim$0.5-3 $\times$M$_*$/16M$_\odot$ au with asymmetric upper layers, revealing that the energy from the accretion burst is deposited at scales below 0.5 au. Some line profiles have redshifted asymmetries, but the system is unlikely sustained by magnetospheric accretion, especially in outburst. The accretion-related structures extend over several stellar radii and, like the wind, are likely non-axisymmetric. The stellar mass may be $\sim$6-8 M$_\odot$, lower than previously thought ($\sim$16 M$_\odot$). Emission line analysis is found to be a powerful tool to study the innermost regions and accretion in stars within a very large range of effective temperatures. The density ranges in the disk and accretion structures are higher than in late-type stars, but the overall behavior, including the innermost disk emission and variable wind, is very similar independently of the spectral type. Our work suggests a common outburst behavior for stars with spectral types ranging from M-type to intermediate-mass stars.Comment: Accepted by A&A, 21 pages plus online appendix. Version 2: Minor typos corrected. Version 3: Note on DIBs added, typos correcte

Identification and spectroscopic characterization of 128 new Herbig stars

We present optical spectroscopy observations of 145 high-mass pre-main sequence candidates from the catalogue of Vioque et al. (2020). From these, we provide evidence for the Herbig nature of 128 sources. This increases the number of known objects of the class by $\sim50\%$. We determine the stellar parameters of these sources using the spectra and Gaia EDR3 data. The new sources are well distributed in mass and age, with 23 sources between $4$-$8$ M$_{\odot}$ and 32 sources above $8$ M$_{\odot}$. Accretion rates are inferred from H$\alpha$ and H$\beta$ luminosities for 104 of the new Herbigs. These accretion rates, combined with previous similar estimates, allow us to analyze the accretion properties of Herbig stars using the largest sample ever considered. We provide further support to the existence of a break in accretion properties at $\sim3$-$4$ M$_{\odot}$, which was already reported for the previously known Herbig stars. We re-estimate the potential break in accretion properties to be at $3.87^{+0.38}_{-0.96}$ M$_{\odot}$. As observed for the previously known Herbig stars, the sample of new Herbig stars independently suggests intense inner-disk photoevaporation for sources with masses above $\sim7$ M$_{\odot}$. These observations provide robust observational support to the accuracy of the Vioque et al. (2020) catalogue of Herbig candidates.Comment: Published in The Astrophysical Journal. Received 1 January 2022 / Accepted 2 March 2022 (14 pages, 8 figures). Tables B1, B2, and B3 are available at the CDS in their entirety in machine-readable for