Now showing 1 - 5 of 5
  • Publication
    Measuring the ionisation fraction in a jet from a massive protostar
    It is important to determine if massive stars form via disc accretion, like their low-mass counterparts. Theory and observation indicate that protostellar jets are a natural consequence of accretion discs and are likely to be crucial for removing angular momentum during the collapse. However, massive protostars are typically rarer, more distant and more dust enshrouded, making observational studies of their jets more challenging. A fundamental question is whether the degree of ionisation in jets is similar across the mass spectrum. Here we determine an ionisation fraction of ~5–12% in the jet from the massive protostar G35.20-0.74N, based on spatially coincident infrared and radio emission. This is similar to the values found in jets from lower-mass young stars, implying a unified mechanism of shock ionisation applies in jets across most of the protostellar mass spectrum, up to at least ~10 solar masses.
    Scopus© Citations 15  405
  • 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.
    Scopus© Citations 11  237
  • 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
      327Scopus© Citations 19
  • Publication
    Parsec-scale jets driven by high-mass young stellar objects: Connecting the au- and the parsec-scale jet in IRAS 13481-6124
    Context. Protostellar jets in high-mass young stellar objects (HMYSOs) play a key role in the understanding of star formation and provide us with an excellent tool to study fundamental properties of HMYSOs. Aims. We aim at studying the physical and kinematic properties of the near-infrared (NIR) jet of IRAS 13481-6124 from au to parsec scales. Methods. Our study includes NIR data from the Very Large Telescope instruments SINFONI, CRIRES, and ISAAC. Information about the source and its immediate environment is retrieved with SINFONI. The technique of spectro-astrometry is performed with CRIRES to study the jet on au scales. The parsec-scale jet and its kinematic and dynamic properties are investigated using ISAAC. Results. The SINFONI spectra in H and K bands are rich in emission lines that are mainly associated with ejection and accretion processes. Spectro-astrometry is applied to the Brγ line, and for the first time, to the Brα line, revealing their jet origin with milliarcsecond-scale photocentre displacements (11-15 au). This allows us to constrain the kinematics of the au-scale jet and to derive its position angle (~216°). ISAAC spectroscopy reveals H2 emission along the parsec-scale jet, which allows us to infer kinematic and dynamic properties of the NIR parsec-scale jet. The mass-loss rate inferred for the NIR jet is Mejec ~ 10-4 M⊙ yr-1 and the thrust is P ~ 10-2 M⊙ yr-1 km s-1, which is roughly constant for the formation history of the young star. A tentative estimate of the ionisation fraction is derived for the massive jet by comparing the radio and NIR mass-loss rates. An ionisation fraction 8% is obtained, which means that the bulk of the ejecta is traced by the NIR jet and that the radio jet only delineates a small portion of it.
    Scopus© Citations 10  293
  • Publication
    From parsec to au scales : zooming-in on the formation of high-mass young stellar objects
    (University College Dublin. School of Physics, 2020)
    In this thesis, I investigate the formation of high-mass young stellar objects (HMYSOs) and the properties of their protostellar jets. My first science chapter is a study of the kinematic and dynamic properties of the 20M HMYSO IRAS 13481-6124 (Fedriani et al. 2018). I measured mass-loss and momentum rates on the order of 10^-4M sun yr-1 and 10^-2Msun yr-1 km s-1, respectively. I used Hi emission lines to perform spectro-astrometry to probe the massive protostellar jet at au-scales and connect it with the parsec-scale jet. In the second science chapter, I study the unique case of G35.2-0.74N, a 10M sun HMYSO (Fedriani et al. 2019). I observe, spatially coincident atomic near-infrared (NIR) and ionised radio jet emission. For the first time, the ionisation fraction from a jet driven by a massive protostar has been measured. The value obtained (~10%) is similar to that found in jets driven by low-mass protostars. This suggests a common launching mechanism, i. e., the jet is launched magneto-centrifugally. It was possible to measure mass-loss and momentum rates on the order of 10-5Msun yr-1 and 10-2Msun yr-1 km s-1, respectively, without relying on assumptions on the ionisation fraction. This study also confirms that the ionisation mechanism is related to shocks instead of UV radiation from the central source. The new method used in this paper, which combines NIR and radio measurements, opens up a new way to measure ionisation fraction of jets for both the low- and high-mass regime. Lastly, the jet and the accretion disc of the driving source of the Herbig-Haro object 135/136, IRAS 11101-5829, was investigated in the third science chapter (Fedriani et al. submitted). I found disc emission, for the first time, in this object revealed by the NIR CO overtone bandhead emission at 2.29 - 2.5 microns. By using our own developed LTE model, I infer that the emission is consistent with a relatively warm (T~3000 K) and dense (NCO~10^22 cm-2) disc. Notably, the model also suggests a geometry different from that inferred in the jet, retrieved from both spectral emission lines and imaging. This implies that the disc emission is reflected off the outflow cavity walls. This approach has been used for the first time in this thesis and indicates that to obtain the correct geometry of the system, one needs both high spectral and spatial resolution to avoid retrieving possible erroneous results. The results presented in this thesis show further observational evidence that the formation of high-mass protostars and their jets are a scaled-up version of their low-mass counterparts and that their properties scale with mass. In light of these results, we are then one step closer to confirm that star formation proceeds in a similar fashion independently of mass.