Investigating episodic accretion in a very low-mass young stellar object

Very low-mass Class I protostars have been investigated very little thus far. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied. We investigate accretion variability in IRS54, a Class I very low-mass protostar with a mass of M$_{\star}$ ~ 0.1 - 0.2 M$_{\odot}$. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared ($J$, $H$, and $K$ bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Pa$\beta$ and Br$\gamma$) and outflow-tracing lines (H$_2$ and [FeII] to examine physical properties and kinematics of the object. A large increase in luminosity was found between the 2005 and 2013 epochs of more than 1 magnitude in the $K$ band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate ($\dot{M}_{acc}$) rose by an order of magnitude from ~ 10$^{-8}$ M$_{\odot}$ yr$^{-1}$ to ~ $10^{-7}$ M$_{\odot}$ yr$^{-1}$ between the two early epochs. The visual extinction ($A_V$) has also increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in $A_V$ in tandem with the increase in $\dot{M}_{acc}$ is explained by the lifting up of a large amount of dust from the disc of IRS54, following the augmented accretion and ejection activity in the YSO, which intersects our line of sight due to the almost edge-on geometry of the disc. Because of the strength and timescales involved in this dramatic increase, this event is believed to have been an accretion burst possibly similar to bursts of EXor-type objects. IRS54 is the lowest mass Class I source observed to have an accretion burst of this type, and therefore potentially one of the lowest mass EXor-type objects known so far

From parsec to au scales : zooming-in on the formation of high-mass young stellar objects

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

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.European Commission Horizon 2020European Research CouncilScience Foundation IrelandSTFC Rutherford Fellowshi

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.European Commission Horizon 2020European Research CouncilScience Foundation IrelandSTFC Rutherford FellowshipEuropean Research Counci

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 encounterEuropean Commission Horizon 2020European Research CouncilIrish Research CouncilScience Foundation IrelandEuropean Southern Observatory FellowshipPublished version is permitted without embargo - A

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.European Commission Horizon 2020European Research CouncilScience Foundation IrelandNational Science Foundation (USA)Check citation details - AC2019-09-12 JG: second bitstream (publisher corrections) added; originally submitted as a separate ite

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 encounterEuropean Commission Horizon 2020European Research CouncilIrish Research CouncilScience Foundation IrelandEuropean Southern Observatory FellowshipPublished version is permitted without embargo - A