arXiv subjects

A. J. Frost

Publications and source records attributed to A. J. Frost.

24 records · Page 2Linked to original sources

arXiv2021

Tying the geometrical traits of massive young stellar objects and their discs to a potential evolutionary sequence using infrared observations

Young massive stars influence their surroundings from local to galactic scales, but the observational challenges associated with their distance and embedded nature has, until the recent decade, made high-resolution studies of these objects difficult. In particular, comparative analyses of massive young stellar object (MYSO) discs are currently lacking and our understanding of their evolution is limited. Here, we combine the results of two studies with the aim to attribute geometrical features to an evolutionary sequence for a sample of seven MYSOs. The time evolution is based on a near-IR spectral features, while the geometry is determined from a multi size-scale study of MYSOs. We find that MYSO discs with determined geometrical substructure turn out to be also spectroscopically more evolved. This implies that disc evolution and dispersal are occurring within MYSOs, similar to low-mass YSO disc evolution, despite their faster formation timescales.

astro-ph.SR↗

arXiv2021

The first interferometric survey in the K-band of massive YSOs. On the hot dust, ionised gas, and binarity at au scales

Circumstellar discs are essential for high mass star formation, while multiplicity, in particular binarity, appears to be an inevitable outcome since the vast majority of massive stars (> 8 Msun) are found in binaries (up to 100%). We constrain the sizes of the dust and ionised gas (Brgamma) emission of the innermost regions towards a sample of six MYSOs, and provide high-mass binary statistics of young stars at 2-300 au scales using VLTI (GRAVITY, AMBER) observations. We determine the inner radius of the dust emission and place MYSOs with K-band measurements in a size-luminosity diagram for the first time, and compare our findings to T Tauris and Herbig AeBes. We also compare the observed K-band sizes to the sublimation radius predicted by three different disc scenarios. Lastly, we apply binary geometries to trace close binarity among MYSOs. The inner sizes of MYSOs, Herbig AeBe and T Tauri stars appear to follow a universal trend at which the sizes scale with the square-root of the stellar luminosity. The Brgamma emission originates from somewhat smaller and co-planar area compared to the 2.2 μm continuum emission. We discuss this new finding with respect to disc-wind or jet origin. Finally, we report an MYSO binary fraction of 17-25% at milli-arcsecond separations (2-300 au). The size-luminosity diagram indicates that the inner regions of discs around young stars scale with luminosity independently of the stellar mass. At the targeted scales (2-300 au), the MYSO binary fraction is lower than what was previously reported for the more evolved main sequence massive stars, which, if further confirmed, could implicate the predictions from massive binary formation theories. Lastly, we spatially resolve the crucial star/disc interface in a sample of MYSOs, showing that au-scale discs are prominent in high-mass star formation and similar to their low-mass equivalents.

astro-ph.SR↗

arXiv2021

Resolving the dynamical mass tension of the massive binary 9 Sagittarii

Direct dynamical mass measurements of stars with masses above 30 M${}_\odot$ are rare. This is the result of the low yield of the upper initial mass function and the limited number of such systems in eclipsing binaries. Long-period, double-lined spectroscopic binaries that are also resolved astrometrically offer an alternative for obtaining absolute masses of stellar objects. 9 Sgr is one such long-period, high-mass binary. Unfortunately, a large amount of tension exists between its total dynamical mass inferred from radial velocity measurements and that from astrometric data. We obtained the astrometric orbit from VLTI/PIONIER and VLTI/GRAVITY interferometric measurements. Using archival and new spectroscopy, we performed a grid-based spectral disentangling search to constrain the semi-amplitudes of the radial velocity curves. We computed atmospheric parameters and surface abundances by adjusting \textsc{fastwind} atmosphere models and we compared our results with evolutionary tracks computed with the Bonn Evolutionary Code (BEC). Grid spectral disentangling of 9 Sgr supports the presence of a 53 M${}_\odot$ primary and a 39 M${}_\odot$ secondary. Comparison with BEC evolutionary tracks shows the components of 9 Sgr are most likely coeval with an age of roughly 1 Myr. Our analysis clears up the contradiction between mass and orbital inclination estimates reported in previous studies. We detect the presence of significant CNO-processed material at the surface of the primary, suggesting enhanced internal mixing compared to currently implemented in the BEC models. The present measurements provide a high-quality high-mass anchor to validate stellar evolution models and to test the efficiency of internal mixing processes.

astro-ph.SR↗

arXiv2021

Unveiling the traits of massive young stellar objects through a multi-scale survey

The rarity and deeply embedded nature of stars with masses larger than 8 solar masses has limited our understanding of their formation. Previous work has shown that complementing spectral energy distributions with interferometric and imaging data can probe the circumstellar environments of massive young stellar objects (MYSOs) well. However, complex studies of single objects often use different approaches in their analysis. Therefore the results of these studies cannot be directly compared. This work aims to obtain the physical characteristics of a sample of MYSOs at ~0.01" scales, at 0.1" scales, and as a whole, which enables us to compare the characteristics of the sources. We apply the same multi-scale method and analysis to a sample of MYSOs. High-resolution interferometric data, near-diffraction-limited imaging data, and a multi-wavelength spectral energy distribution are combined. By fitting simulated observables derived from 2.5D radiative transfer models of disk-outflow-envelope systems to our observations, the properties of the MYSOs are constrained. We find that the observables of all the MYSOs can be reproduced by models with disk-outflow-envelope geometries, analogous to the Class I geometry associated with low-mass protostars. The characteristics of the envelopes and the cavities within them are very similar across our sample. On the other hand, the disks seem to differ between the objects, in particular with regards to what we interpret as evidence of complex structures and inner holes. This is comparable to the morphologies observed for low-mass young stellar objects. A strong correlation is found between the luminosity of the central MYSO and the size of the transition disk-like inner hole for the MYSOs, implying that photoevaporation or the presence of binary companions may be the cause.

astro-ph.SR↗

arXiv2020

Is HR 6819 a triple system containing a black hole? -- An alternative explanation

HR 6819 was recently proposed to be a triple system consisting of an inner B-type giant + black hole binary with an orbital period of 40d and an outer Be tertiary. This interpretation is mainly based on two inferences: that the emission attributed to the outer Be star is stationary, and that the inner star, which is used as mass calibrator for the black hole, is a B-type giant. We re-investigate the properties of HR 6819 by spectral disentangling and an atmosphere analysis of the disentangled spectra to search for a possibly simpler alternative explanation for HR 6819. Disentangling implies that the Be component is not a static tertiary, but rather a component of the binary in the 40-d orbit. The inferred radial velocity amplitudes imply an extreme mass ratio of M_2/M_1 = 15 +/- 3. We infer spectroscopic masses of 0.4$^{+0.3}_{-0.1}$ Msun and 6$^{+5}_{-3}$ Msun for the primary and secondary, which agree well with the dynamical masses for an inclination of i = 32 deg. This indicates that the primary might be a stripped star rather than a B-type giant. Evolutionary modelling suggests that a possible progenitor system would be a tight (P_i ~ 2d) B+B binary system that experienced conservative mass transfer. While the observed nitrogen enrichment of the primary conforms with the predictions of the evolutionary models, we find no indications for the predicted He enrichment. We suggest that HR 6819 is a binary system consisting of a stripped B-type primary and a rapidly-rotating Be star that formed from a previous mass-transfer event. In the framework of this interpretation, HR 6819 does not contain a black hole. Interferometry can distinguish between these two scenarios by providing an independent measurement of the separation between the visible components.

astro-ph.SR↗

arXiv2019

A multi-scale exploration of a massive young stellar object - a transition disk around G305.20+0.21?

The rarity of young massive stars combined with the fact that they are often deeply embedded has limited the understanding of their formation. Ground based mid-infrared (IR) interferometry is one way of securing the spatial resolution required to study massive young stellar objects (MYSOs) and as the spatial-frequency coverage of such observations is often incomplete, direct-imaging can be supplementary to such a dataset. By consolidating these observations with modelling, the features of a massive protostellar environment can be constrained. This work simultaneously fits the aforementioned observations and a spectral energy distribution (SED) with a 2.5D radiative transfer model, providing an extensive view of the physical characteristics of the accreting regions of the MYSO G305.20+0.21. The high-resolution observations were obtained using the Very Large Telescope's MIDI and VISIR instruments, producing visibilities in the N-band and near-diffraction-limited imaging in the Q-band respectively. A model including a central protostar with a luminosity of $\sim$5$\times$10$^{4}$L$_{\odot}$ surrounded by a low-density bipolar cavity, a flared 1M$_{\odot}$ disk and an envelope provides a sufficient fit all three types of observation. The need to include a disk in the model implies that this MYSO follows a scaled-up version of the low-mass star formation process. The weak silicate absorption feature within the SED requires low-density envelope cavities to be successfully fit and is an atypical characteristic in comparison to previously studied MYSOs. Additionally, the inner radius of the disk must be three times the dust sublimation radius to satisfy the MIDI visibilities. The low density, low extinction environment implies the object is a more evolved MYSO and this combined with large inner radius of the disk suggests that it could be an example of a transitional disk around an MYSO.

astro-ph.SR↗