Medium-resolution near-infrared spectroscopy of massive young stellar objects

Pomohaci, Robert; Oudmaijer, R. D.; Lumsden, S. L.; Hoare, M. G.; Mendigutía, I.

doi:10.1093/mnras/stx2196

Cited by 22 publications

(42 citation statements)

References 56 publications

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“…However, Simon et al (1983) showed that winds of MYSOs can be so dense that hydrogen is collisionally excited to its n=2 state, which makes its ionisation from cooler stars possible (see also, Koumpia et al 2020;Drew et al 1998;Drew 1998). Indeed, a bloated star could also explain the narrow single line profiles of Brγ observed in MYSOs in various studies (e.g., Pomohaci et al 2017), which is in contrast to the relatively broader and double-peaked lines predicted by disc models around hot main sequence stars (Sim et al 2005). Indeed, a slightly bloated object would have a larger disc (inner) radius and thus lower rotational velocities (i.e., narrow line profiles).…”

Section: Origin Of the Brγ Emissionmentioning

confidence: 87%

“…The origin of the Brγ emission observed in the close vicinity of (M)YSOs (Mendigutía et al 2011;Murakawa et al 2013;Davies et al 2010;Kraus et al 2008;Pomohaci et al 2017;Lumsden et al 2012) can be explained by theory (e.g., jets, magnetospheric accretion, disc wind; Ferreira 1997;Tambovtseva et al 2014Tambovtseva et al , 2016 and can be further tested by observationally constraining the spatial origin of the emission compared to the size of the hot dust (2.2 µm emission). The GRAVITY dataset allows us to estimate the size of the ionised gas by measuring visibilities at the central spectral channel of the Brγ emission towards 4 MYSOs and directly compare it to that of the continuum emission.…”

Section: Brγ Emissionmentioning

confidence: 99%

“…In Pomohaci et al (2017), it is argued than when dealing with MYSOs and sources which are characterised by large continuum excess, the contribution of the photospheric absorption is negligible.…”

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confidence: 99%

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The first interferometric survey of massive YSOs in the K-band

Koumpia

Wit

Oudmaijer

et al. 2021

A&A

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Context. Circumstellar discs are essential for the formation of high mass stars, while multiplicity, and in particular binarity, appears to be an inevitable outcome, as the vast majority of massive stars (>8 M⊙) are found in binaries (up to 100%). Our understanding of the innermost regions of accretion discs around massive stars and the binarity of high-mass young stars is sparse because of the high spatial resolution and sensitivity required to trace these rare and distant objects. Aims. We aim to spatially resolve and constrain the sizes of the dust and ionised gas emission from the innermost regions of a sample of massive young stellar objects (MYSOs) for the first time, and to provide high-mass binary statistics for young stars at 2–300 au scales using direct interferometric measurements. Methods. We observed six MYSOs using long-baseline near-infrared K-band interferometry on the VLTI (GRAVITY, AMBER) in order to resolve and characterise the 2.2 μm hot dust emission originating from the inner rim of circumstellar discs around MYSOs, and the associated Brγ emission from ionised gas. We fitted simple geometrical models to the interferometric observables, and determined the inner radius of the dust emission. We placed MYSOs with K-band measurements in a size–luminosity diagram for the first time, and compared our findings to their low- and intermediate-mass counterparts (T Tauris and Herbig AeBes). We also compared the observed K-band sizes (i.e. inner rim radius) to the sublimation radius predicted by three different disc scenarios: a classical thick flattened structure with oblique heating in action, and direct heating from the protostar via an optically thin cavity with and without backwarming effects. Lastly, we applied binary geometries to trace close binarity among MYSOs. Results. The characteristic size of the 2.2 μm continuum emission towards this sample of MYSOs shows a large scatter at the given luminosity range. When the inner sizes of MYSOs are compared to those of lower mass Herbig AeBe and T Tauri stars, they appear to follow a universal trend in that the sizes scale with the square-root of the stellar luminosity. The Brγ emission originates from a similar or somewhat smaller and co-planar area compared to the 2.2 μm continuum emission. We discuss this new finding with respect to a disc-wind or jet origin. Finally, we report an MYSO binary fraction of 17–25% at milli-arcsecond separations (2–300 au). Conclusions. The size–luminosity diagram indicates that the inner regions of discs around young stars scale with luminosity independently of the stellar mass. The observed fraction of MYSO binaries in K-band is almost ‘flat’ for a wide range of separations (2–10 000 au). 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 predictions from massive binary formation theories. Lastly, with this study, we can finally 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 are similar to their low-mass equivalents, while the ionised gas can be linked to disc wind and disc accretion models similar to Herbig AeBes.

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Section: Origin Of the Brγ Emissionmentioning

confidence: 87%

Section: Brγ Emissionmentioning

confidence: 99%

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The first interferometric survey of massive YSOs in the K-band

Koumpia

Wit

Oudmaijer

et al. 2021

A&A

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“…Only recently this has been achieved for the first time. The MYSO G015.1288-00.6717 shows an A-type spectrum with a relatively low surface gravity, pointing to a bloated protostar (Pomohaci et al 2017).…”

Section: Uncertainty Of the Accretion Parametersmentioning

confidence: 99%

Infrared observations of the flaring maser source G358.93−0.03

Stecklum

Wolf

Linz

et al. 2021

A&A

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Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent evidence shows that flares of these masers are driven by MYSO accretion bursts. Thus, maser monitoring can be used to identify such bursts which are hard to discover otherwise. Infrared observations reveal burst-induced changes in the spectral energy distribution (first and foremost a luminosity increase), which provide valuable information on a very intense phase of high-mass star formation. Aims. In mid-January 2019, flaring of the 6.7 GHz CH3OH maser (hereafter maser) of the MYSO G358.93-0.03 (hereafter G358) was reported. The international maser community initiated an extensive observational campaign which revealed extraordinary maser activity and yielded the detection of numerous new masering transitions. Interferometric imaging with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array resolved the maser emitting core of the star forming region and proved the association of the masers with the brightest continuum source (MM1), which hosts a hot molecular core. These observations, however, failed to detect a significant rise in the (sub)millimeter dust continuum emission. Therefore, we performed near-infrared (NIR) and far-infrared (FIR) observations to prove or disprove whether the CH3OH flare was driven by an accretion burst. Methods. NIR imaging with the Gamma-Ray Burst Optical/Near-infrared Detector has been acquired and integral-field spectroscopy with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) was carried out on two occasions to detect possible counterparts to the (sub)millimeter sources and compare their photometry to archival measurements. The comparison of pre-burst and burst spectral energy distributions is of crucial importance to judge whether a substantial luminosity increase, caused by an accretion burst, is present and if it triggered the maser flare. Radiative transfer modeling of the spectral energy distribution (SED) of the dust continuum emission at multiple epochs provides valuable information on the bursting MYSO. Results. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our radiative transfer modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the pre-burst, burst, and post-burst SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. Conclusions. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. Thus, monitoring of these masers greatly enhances the chances of identifying MYSOs during periods of intense growth. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous accretion burst known to date. According to models, bursts of this kind occur most often.

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“…High-mass young objects tend to have a larger fraction of CO detections than lower luminosity ones. Most recently, Pomohaci et al (2017) detected the CO overtone emission in a subset of 7 out of 38 massive young stellar objects (MYSO). These detections seem to be confined to a narrow range ofṀ acc (10 −4.5 -10 −3.0 M yr −1 ), where they account for about 40% of the objects.…”

Section: Discussionmentioning

confidence: 99%

The GRAVITY young stellar object survey

Koutoulaki

Lopez²,

Natta³

et al. 2021

A&A

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Context. 51 Oph is a Herbig Ae/Be star that exhibits strong near-infrared CO ro-vibrational emission at 2.3 μm, most likely originating in the innermost regions of a circumstellar disc. Aims. We aim to obtain the physical and geometrical properties of the system by spatially resolving the circumstellar environment of the inner gaseous disc. Methods. We used the second-generation Very Large Telescope Interferometer instrument GRAVITY to spatially resolve the continuum and the CO overtone emission. We obtained data over 12 baselines with the auxiliary telescopes and derive visibilities, and the differential and closure phases as a function of wavelength. We used a simple local thermal equilibrium ring model of the CO emission to reproduce the spectrum and CO line displacements. Results. Our interferometric data show that the star is marginally resolved at our spatial resolution, with a radius of ~10.58 ± 2.65R⊙. The K-band continuum emission from the disc is inclined by 63° ± 1°, with a position angle of 116° ± 1°, and 4 ± 0.8 mas (0.5 ± 0.1 au) across. The visibilities increase within the CO line emission, indicating that the CO is emitted within the dust-sublimation radius. By modelling the CO bandhead spectrum, we derive that the CO is emitted from a hot (T = 1900–2800 K) and dense (NCO = (0.9–9) × 1021 cm−2) gas. The analysis of the CO line displacement with respect to the continuum allows us to infer that the CO is emitted from a region 0.10 ± 0.02 au across, well within the dust-sublimation radius. The inclination and position angle of the CO line emitting region is consistent with that of the dusty disc. Conclusions. Our spatially resolved interferometric observations confirm the CO ro-vibrational emission within the dust-free region of the inner disc. Conventional disc models exclude the presence of CO in the dust-depleted regions of Herbig AeBe stars. Ad hoc models of the innermost disc regions, that can compute the properties of the dust-free inner disc, are therefore required.

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Medium-resolution near-infrared spectroscopy of massive young stellar objects

Cited by 22 publications

References 56 publications

The first interferometric survey of massive YSOs in the K-band

The first interferometric survey of massive YSOs in the K-band

Infrared observations of the flaring maser source G358.93−0.03

The GRAVITY young stellar object survey

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