AMBER and CRIRES observations of the binary sgB[e] star HD 327083: evidence of a gaseous disc traced by CO bandhead emission

HD 327083 is a sgB[e] star that forms a binary system with an orbital semi-major axis of ~1.7 AU. Our previous observations using the VLTI and AMBER in the medium resolution K-band mode spatially resolved the environment of HD 327083. The continuum visibilities obtained indicate the presence of a circumbinary disc. CO bandhead emission was also observed. However, due to the limited spectral resolution of the previous observations, the kinematic structure of the emitting material was not constrained. In this paper, we address this and probe the source of the CO emission with high spectral resolution and spatial precision. We have observed HD 327083 with high spectral resolution (25 & 6 km/s) using AMBER and CRIRES. The observations are compared to kinematical models to constrain the source of the emission. It is shown that the CO bandhead emission can be reproduced using a model of a Keplerian disc with an inclination and size consistent with our previous VLTI observations. The model is compared to AMBER differential phase measurements, which have a precision as high as 30-micro-arcseconds. A differential phase signal corresponding to 0.15 milli-arcseconds (~5 sigma) is seen over the bandhead emission, which is in excellent agreement with the model that fits the CRIRES observations. In comparison, a model of an equatorial outflow, as envisaged in the standard sgB[e] scenario, does not reproduce the observations well. The excellent agreement between the disc model and observations in the spatial and spectral domains is compelling evidence that the CO bandhead emission of HD 327083 originates in a circumbinary Keplerian disc. In contrast, the model of an equatorial outflow cannot reproduce the observations well. This suggests that the standard sgB[e] scenario is not applicable to HD 327083, which supports the hypothesis that the B[e] behaviour of HD 327083 is due to binarity (ABRIDGED).Comment: Accepted for publication in A&

Near-infrared integral field spectroscopy of Massive Young Stellar Objects

We present medium resolution ($R\approx5300$) $K$-band integral field spectroscopy of six MYSOs. The targets are selected from the RMS survey, and we used the NIFS on the Gemini North telescope. The data show various spectral line features including Br$\gamma$, CO, H$_2$, and \mbox{He\,{\sc i}}. The Br$\gamma$ line is detected in emission in all objects with $v_\mathrm{FWHM}\sim100$ -- 200 kms$^{-1}$. V645 Cyg shows a high-velocity P-Cygni profile between -800 kms$^{-1}$ and -300 kms$^{-1}$. We performed three-dimensional spectroastrometry to diagnose the circumstellar environment in the vicinity of the central stars using the Br$\gamma$ line. We measured the centroids of the velocity components with sub-mas precision. The centroids allow us to discriminate the blueshifted and redshifted components in a roughly east--west direction in both IRAS 18151--1208 and S106 in Br$\gamma$. This lies almost perpendicular to observed larger scale outflows. We conclude, given the widths of the lines and the orientation of the spectroastrometric signature, that our results trace a disc wind in both IRAS 18151--1208 and S106. The CO $\nu=2-0$ absorption lines at low $J$ transitions are detected in IRAS 18151--1208 and AFGL 2136. We analysed the velocity structure of the neutral gas discs. In IRAS 18151--1208, the absorption centroids of the blueshifted and redshifted components are separated in a direction of north-east to south-west, nearly perpendicular to that of the larger scale $H_2$ jet. The position-velocity relations of these objects can be reproduced with central masses of 30 M_{\sun} for IRAS 18151--1208 and 20 M_{\sun} for AFGL 2136. We also detect CO $\nu=2-0$ bandhead emission in IRAS 18151--1208, S106 and V645 Cyg. The results can be fitted reasonably with a Keplerian rotation model, with masses of 15, 20 and 20 M_{\sun} respectively.Comment: 17 pages, 10 figures, accepted by MNRA

Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs

The formation and earliest stages of protoplanetary discs remain poorly constrained by observations. ALMA will soon revolutionise this field. Therefore, it is important to provide predictions which will be valuable for the interpretation of future high sensitivity and high angular resolution observations. Here we present simulated ALMA observations based on radiative transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc embedded in a 10 M_solar dense core, with structure similar to the pre-stellar core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2, OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA

Self-gravitating disc candidates around massive young stars

DHF gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG. JDI gratefully acknowledges support from the DISCSIM project, grant agreement 341137, funded by the European Research Council under ERC-2013-ADG. CJC acknowledges support from STFC grant ST/M001296/1.There have been several recent detections of candidate Keplerian discs around massive young protostars. Given the relatively large disc-to-star mass ratios in these systems, and their young ages, it is worth investigating their propensity to becoming self-gravitating. To this end, we compute self-consistent, semi-analytic models of putative self-gravitating discs for five candidate disc systems. Our aim is not to fit exactly the observations, but to demonstrate that the expected dust continuum emission from marginally unstable self-gravitating discs can be quite weak, due to high optical depth at the mid-plane even at millimetre wavelengths. In the best cases, the models produce ‘observable’ disc masses within a factor of <1.5 of those observed, with mid-plane dust temperatures comparable to measured temperatures from molecular line emission. We find in two cases that a self-gravitating disc model compares well with observations. If these discs are self-gravitating, they satisfy the conditions for disc fragmentation in their outer regions. These systems may hence have as-yet-unresolved low-mass stellar companions, and are thus promising targets for future high angular resolution observations.PostprintPeer reviewe

Discovery of a 500 au protobinary in the massive prestellar core G11.92-0.61 MM2

Funding: C.J.C. acknowledges support from the University of St Andrews Restarting Research Funding Scheme (SARRF), which is funded through the SFC grant reference SFC/AN/08/020. J.D.I. acknowledges support from the UK’s STFC under ST/T000287/1. S.Z. is funded by the China Scholarship Council–University of St Andrews Scholarship (PhD programmes, No. 201806190010). T.J.H. is funded by a Royal Society Dorothy Hodgkin Fellowship.We present high-resolution ( 24.7 L⊙ for MM2E and L* > 12.6 L⊙ for MM2W. The compact sources are connected by a "bridge" of lower-surface-brightness dust emission and lie within more extended emission that may correspond to a circumbinary disk. The circumprotostellar gas mass, estimated from ~0.2" resolution VLA 0.9 cm observations assuming optically thin emission, is 6.8 ± 0.9 M⊙. No line emission is detected towards MM2E and MM2W in our high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J=2-1, in absorption against the 1.3 mm continuum, which likely traces a layer of cooler molecular material surrounding the protostars. We also report the discovery of a highly asymmetric bipolar molecular outflow that appears to be driven by MM2E and/or MM2W in new deep, ~0.5" resolution (1680 au) ALMA 0.82 mm observations. This outflow, traced by low-excitation CH3OH emission, indicates ongoing accretion onto the protobinary system. Overall, the super-Alfvenic models of Mignon-Risse et al. (2021) agree well with the observed properties of the MM2E/MM2W protobinary, suggesting that this system may be forming in an environment with a weak magnetic field.Publisher PDFPeer reviewe

Blinded by the light: on the relationship between CO first overtone emission and mass accretion rate in massive young stellar objects

To date, there is no explanation as to why disc-tracing CO first overtone (or ‘bandhead’) emission is not a ubiquitous feature in low- to medium-resolution spectra of massive young stellar objects (MYSOs), but instead is only detected towards approximately 25 per cent of their spectra. In this paper, we investigate the hypothesis that only certain mass accretion rates result in detectable bandhead emission in the near-infrared spectra of MYSOs. Using an analytic disc model combined with an LTE model of the CO emission, we find that high accretion rates (≳10⁻⁴ M⊙ yr⁻¹) result in large dust sublimation radii, a larger contribution to the K-band continuum from hot dust at the dust sublimation radius, and therefore correspondingly lower CO emission with respect to the continuum. On the other hand, low accretion rates (≲10⁻⁶ M⊙ yr⁻¹) result in smaller dust sublimation radii, a correspondingly smaller emitting area of CO, and thus also lower CO emission with respect to the continuum. In general, moderate accretion rates produce the most prominent, and therefore detectable, CO first overtone emission. We compare our findings to a recent near-infrared spectroscopic survey of MYSOs, finding results consistent with our hypothesis. We conclude that the detection rate of CO bandhead emission in the spectra of MYSOs could be the result of MYSOs exhibiting a range of mass accretion rates, perhaps due to the variable accretion suggested by recent multi-epoch observations of these objects

LBT/LUCIFER near-infrared spectroscopy of PV Cephei. An outbursting young stellar object with an asymmetric jet

We present a detailed spectroscopic investigation of the young eruptive star PV Cep, to improve our understanding of its nature and characterise its circumstellar environment after its last outburst in 2004. The analysis of our medium-resolution spectroscopy in the near-IR (0.9-2.35 um), collected in 2012 at the Large Binocular Telescope with the IR spectrograph LUCIFER, allows us to infer the main stellar parameters (visual extinction, accretion luminosity, mass accretion and ejection rates), and model the inner disc, jet, and wind. The NIR spectrum displays several strong emission lines associated with accretion/ejection activity and circumstellar environment. Our analysis shows that the brightness of PV Cep is fading, as well as the mass accretion rate (2x10^-7 Msun/yr^-1 in 2012 vs ~5x10^-6 Msun/yr^-1 in 2004), which is more than one order of magnitude lower than in the outburst phase. Among the several emission lines, only the [FeII] intensity increased after the outburst. The observed [FeII] emission delineates blue- and red-shifted lobes, both with high- and low-velocity components, which trace an asymmetric jet and wind, respectively. The observed emission in the jet has a dynamical age of ~8 years, indicating that it was produced during the last outburst. The mass ejection rate in both lobes is 1.5x10^-7 Msun/yr^-1, approximately matching the high accretion rate observed during and immediately after the outburst . The observed jet/outflow asymmetries are consistent with an inhomogeneous medium. Our modelling of the CO emission hints at a small-scale gaseous disc ring, extending from ~0.2-0.4 AU to ~3 AU from the source, with an inner temperature of ~3000 K. Our HI lines modelling indicates that most of the observed emission comes from an expanding disc wind at Te=10000 K. The line profiles are strongly affected by scattering, disc screening, and outflow self-absorption.Comment: To be published in A&

High-resolution Br γ spectro-interferometry of the transitional Herbig Ae/Be star HD 100546: a Keplerian gaseous disc inside the inner rim

We present spatially and spectrally resolved Br γ emission around the planet-hosting, transitional Herbig Ae/Be star HD 100546. Aiming to gain insight into the physical origin of the line in possible relation to accretion processes, we carried out Br γ spectro-interferometry using AMBER/VLTI from three different baselines achieving spatial and spectral resolutions of 2–4 mas and 12 000. The Br γ visibility is larger than that of the continuum for all baselines. Differential phases reveal a shift between the photocentre of the Br γ line – displaced ∼0.6 mas (0.06 au at 100 pc) NE from the star – and that of the K-band continuum emission – displaced ∼0.3 mas NE from the star. The photocentres of the redshifted and blueshifted components of the Br γ line are located NW and SE from the photocentre of the peak line emission, respectively. Moreover, the photocentre of the fastest velocity bins within the spectral line tends to be closer to that of the peak emission than the photocentre of the slowest velocity bins. Our results are consistent with a Br γ-emitting region inside the dust inner rim ( ≲ 0.25 au) and extending very close to the central star, with a Keplerian, disc-like structure rotating counter-clockwise, and most probably flared (∼25°). Even though the main contribution to the Br γ line does not come from gas magnetically channelled on to the star, accretion on to HD 100546 could be magnetospheric, implying a mass accretion rate of a few 10−7 M⊙ yr−1. This value indicates that the observed gas has to be replenished on time-scales of a few months to years, perhaps by planet-induced flows from the outer to the inner disc as has been reported for similar systems

Observing substructure in circumstellar discs around massive young stellar objects

MRJ is funded by the President’s PhD scholarship of the Imperial College London and the ‘Dositeja’ stipend from the Fund for Young Talents of the Serbian Ministry for Youth and Sport. TJH is funded by an Imperial College Junior Research Fellowship. JDI gratefully acknowledges support from the DISCSIM project, grant agreement 341137, funded by the European Research Council under ERC-2013-ADG and support from the STFC (grant number ST/R000549/1). DF gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG. CJC acknowledges support from the STFC (grant number ST/M001296/1). CW acknowledges financial support from the University of Leeds.Simulations of massive star formation predict the formation of discs with significant substructure, such as spiral arms and clumps due to fragmentation. Here, we present a semi-analytic framework for producing synthetic observations of discs with substructure, in order to determine their observability in interferometric observations. Unlike post-processing of hydrodynamical models, the speed inherent to our approach permits us to explore a large parameter space of star and disc parameters, and thus constrain properties for real observations. We compute synthetic dust continuum and molecular line observations probing different disc masses, distances, inclinations, thermal structures, dust distributions, and number and orientation of spirals and fragments. With appropriate spatial and kinematic filtering applied, our models predict that Atacama Large Millimetre Array observations of massive young stellar objects at at <5 kpc distances should detect spirals in both gas and dust in strongly self-gravitating discs (i.e. discs with up to two spiral arms and strong kinematic perturbations). Detecting spirals will be possible in discs of arbitrary inclination, either by directly spatially resolving them for more face-on discs (inclinations up to ~50 deg), or through a kinematic signature otherwise. Clumps resulting from disc fragmentation should be detectable in the continuum, if the clump is sufficiently hotter than the surrounding disc material.PostprintPeer reviewe

Chemistry in a gravitationally unstable protoplanetary disc

Until now, axisymmetric, alpha-disc models have been adopted for calculations of the chemical composition of protoplanetary discs. While this approach is reasonable for many discs, it is not appropriate when self-gravity is important. In this case, spiral waves and shocks cause temperature and density variations that affect the chemistry. We have adopted a dynamical model of a solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc, similar to those that may be found around Class 0 and early Class I protostars, in a study of disc chemistry. We find that for each of a number of species, e.g. H2O, adsorption and desorption dominate the changes in the gas-phase fractional abundance; because the desorption rates are very sensitive to temperature, maps of the emissions from such species should reveal the locations of shocks of varying strengths. The gas-phase fractional abundances of some other species, e.g. CS, are also affected by gas-phase reactions, particularly in warm shocked regions. We conclude that the dynamics of massive discs have a strong impact on how they appear when imaged in the emission lines of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA