Examinando por Autor "Wichittanakom, C."

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Acceso Abierto
K-band GRAVITY/VLTI interferometry of “extreme” Herbig Be stars. The size–luminosity relation revisited
(EDP Sciences, 2021-08-11) Marcos Arenal, P.; Mendigutía, I.; Koumpia, E.; Oudmaijer, R. D.; Vioque, M.; Guzmán Díaz, J.; Wichittanakom, C.; De Wit, W. J.; Montesinos, B.; Ilee, J. D.; Agencia Estatal de Investigación (AEI), European Research Council; Comunidad de Madrid; Science and Technology Facilities Council (STFC); European Research Council (ERC); Marcos Arenal, P. [0000-0003-1549-9396]
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.
Acceso Abierto
The accretion rates and mechanisms of Herbig Ae/Be stars
(Oxford Academics: Oxford University Press, 2020-01-20) Wichittanakom, C.; Oudmaijer, R. D.; Fairlamb, J. R.; Mendigutía, I.; Vioque, M.; Ababakr, K. M.; Comunidad de Madrid; European Research Council (ERC); National Science Foundation (NSF); Mendigutía, I. [0000-0002-0233-5328]; Farlamb, J. [0000-0002-2833-2344]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
This work presents a spectroscopic study of 163 Herbig Ac/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 Ha emission line measurements. Our data is complemented with the X-Shooter sample from previous studies and we update results using Gain 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 to be 3.98(-0.94)(+1.37) M-circle dot. 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 the mass 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.