Now showing 1 - 4 of 4
  • Publication
    A closer look at protoplanetary discs : the inner few au probed with spectroscopy and optical interferometry
    (University College Dublin. School of Physics, 2020)
    A first step towards understanding planetary formation is the characterisation of the structure and evolution of protoplanetary discs. Although the large scale disc is understood in some detail, much less is known about the inner 5 au in which the main physical processes take place: accretion, ejection, and planetary formation. Even in the nearest sites of star formation, this region cannot be spatially resolved by stand-alone telescopes; only in recent years have optical and infrared interferometers been able to achieve this, and only in the case of the brightest sources. Hydrogen recombination lines are present in the spectra of young stars. In particular, the Bracket gamma line is the brightest line in the K-band. HD50138 is a target that exhibits bright Bracket gamma and faint high-n Pfund (from level n=24 to n=19) emission. Our results show that the circumstellar environment of the target is complex and shows asymmetries in both the Bracket gamma and the continuum emitting regions. The Bracket gamma is more compact than the continuum emitting region and originates from a wind with a wide opening angle. The origin of the Bracket gamma line is still controversial and it has been found to be associated with both accretion and ejection processes. Thus, more tracers are needed to better constrain the inner gaseous disc properties. Molecules are also present in this part of the disc. In particular, the CO overtone emission at 2.3\,\mic\ is a good tracer of the inner disc properties. This emission is not common and in the case of T Tauri stars it has been observed in only three sources. RW Aur A is a CTTS that exhibits irregular dimming events. It shows CO overtone emission in both the bright and dim states. Modelling this emission with an LTE CO ring in Keplerian rotation, we find that the CO conditions do not change during the dimming events. Moreover, no significant accretion variation is detected between the two states, leading to the conclusion that the dimmings are caused by occulting material very close to the star (within 1 au). The CO ro-vibrational emission at 2.3\,\mic\ observed with the second generation interferometer VLTI/GRAVITY opens a new window on constraining the physical properties of the inner gaseous disc, by spatially resolving the CO emission. 51Oph is a target which shows bright CO overtone emission. By modelling the CO emission, the CO is found to emitt very close to the star from a warm and dense gas. This is the first observational evidence of the physical properties so close to the star (~0.1 au) where there is no dust. This is consistent with previous LTE models of the physical conditions in the dust-free disc.
  • Publication
    The circumstellar environment of HD 50138 revealed by VLTI/AMBER at high angular resolution
    Context. HD 50138 is a Herbig B[e] star with a circumstellar disc detected at infrared and millimeter wavelength. Its brightness makes it a good candidate for near-infrared interferometry observations. Aims. We aim to resolve, spatially and spectrally, the continuum and hydrogen emission lines in the 2.12–2.47 micron region, to shed light on the immediate circumstellar environment of the star. Methods. VLTI/AMBER K-band observations provide spectra, visibilities, differential phases, and closure phases along three long baselines for the continuum, and H I emission in Brγ and five high-n Pfund lines. By computing the pure line visibilities, we derive the angular size of the different line-emitting regions. A simple local thermodynamic equilibrium (LTE) model was created to constrain the physical conditions of H I emitting region. Results. The continuum region cannot be reproduced by a geometrical two-dimensional (2D) elongated Gaussian fitting model. We estimate the size of the region to be 1 au. We find the detected hydrogen lines (Brγ and Pfund lines) come from a more compact region of size 0.4 au. The Brγ line exhibits an S-shaped differential phase, indicative of rotation. The continuum and Brγ line closure phases show offsets of ~ −25 ± 5° and 20 ± 10° respectively. This is evidence of an asymmetry in their origin, but with opposing directions. We find that we cannot converge on constraints for the HI physical parameters without a more detailed model. Conclusions. Our analysis reveals that HD 50138 hosts a complex circumstellar environment. Its continuum emission cannot be reproduced by a simple disc brightness distribution. Similarly, several components must be evoked to reproduce the interferometric observables within the Brγ line. Combining the spectroscopic and interferometric data of the Brγ and Pfund lines favours an origin in a wind region with a large opening angle. Finally, although we cannot exclude the possibility that HD 50138 is a young star our results point to an evolved source.
      308Scopus© Citations 2
  • Publication
    Mirror, mirror on the outflow cavity wall: Near-infrared CO overtone disc emission of the high-mass YSO IRAS 11101-5829
    Aims. The inner regions of high-mass protostars are often invisible in the near-infrared, obscured by thick envelopes and discs. We aim to investigate the inner gaseous disc of IRAS 11101-5829 through scattered light from the outflow cavity walls. Methods. We observed the immediate environment of the high-mass young stellar object IRAS 11101-5829 and the closest knots of its jet, HH135-136, with the integral field unit VLT/SINFONI. We also retrieved archival data from the high-resolution long-slit spectrograph VLT/X-shooter. We analysed imaging and spectroscopic observations to discern the nature of the near-infrared CO emission. Results. We detect the first three bandheads of the υ = 2−0 CO vibrational emission for the first time in this object. It is coincident with continuum and Brγ emission and extends up to ~10 000 au to the north-east and ~10 000 au to the south-west. The line profiles have been modelled as a Keplerian rotating disc assuming a single ring in local thermodynamic equilibrium. The model output gives a temperature of ~3000 K, a CO column density of ~1 × 1022 cm−2, and a projected Keplerian velocity vK sin idisc ~ 25 km s−1, which is consistent with previous modelling in other high-mass protostars. In particular, the low value of vK sin idisc suggests that the disc is observed almost face-on, whereas the well-constrained geometry of the jet imposes that the disc must be close to edge-on. This apparent discrepancy is interpreted as the CO seen reflected in the mirror of the outflow cavity wall. Conclusions. From both jet geometry and disc modelling, we conclude that all the CO emission is seen through reflection by the cavity walls and not directly. This result implies that in the case of highly embedded objects, as for many high-mass protostars, line profile modelling alone might be deceptive and the observed emission could affect the derived physical and geometrical properties; in particular the inclination of the system can be incorrectly interpreted.
      248Scopus© Citations 11
  • Publication
    Exploring the dimming event of RW Aurigae A through multi-epoch VLT/X-shooter spectroscopy
    Context. RW Aur A is a classical T Tauri star that has suddenly undergone three major dimming events since 2010. The reason for these dimming events is still not clear. Aims. We aim to understand the dimming properties, examine accretion variability, and derive the physical properties of the inner disc traced by the CO ro-vibrational emission at near-infrared wavelengths (2.3 μm). Methods. We compared two epochs of X-shooter observations, during and after the dimming. We modelled the rarely detected CO bandhead emission in both epochs to examine whether the inner disc properties had changed. The spectral energy distribution was used to derive the extinction properties of the dimmed spectrum and compare the infrared excess between the two epochs. Lines tracing accretion were used to derive the mass accretion rate in both states. Results. The CO originates from a region with physical properties of T = 3000 K, NCO = 1 × 1021 cm−2 and vk sin i = 113 km s−1. The extinction properties of the dimming layer were derived with the effective optical depth ranging from τeff ~2.5−1.5 from the UV to the near-IR. The inferred mass accretion rate Ṁacc is ~1.5 × 10−8 M⊙ yr−1 and ~2 × 10−8 M⊙ yr−1 after and during the dimming respectively. By fitting the spectral energy distribution, additional emission is observed in the infrared during the dimming event from dust grains with temperatures of 500–700 K. Conclusions. The physical conditions traced by the CO are similar for both epochs, indicating that the inner gaseous disc properties do not change during the dimming events. The extinction curve is flatter than that of the interstellar medium, and large grains of a few hundred microns are thus required. When we correct for the observed extinction, the mass accretion rate is constant in the two epochs, suggesting that the accretion is stable and therefore does not cause the dimming. The additional hot emission in the near-IR is located at about 0.5 au from the star and is not consistent with an occulting body located in the outer regions of the disc. The dimming events could be due to a dust-laden wind, a severe puffing-up of the inner rim, or a perturbation caused by the recent star-disc encounter
      347Scopus© Citations 19