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Register a new userAn optical parsec-scale jet from a massive young star in the Large Magellanic Cloud
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Authors
A. F. McLeod
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Highly collimated parsec-scale jets, generally linked to the presence of an accretion disk, are a commonly observed phenomenon from revealed low-mass young stellar objects. In the past two decades, only a very few of these objects have been directly (or indirectly) observed towards high-mass (M > 8 M$_{odot}$) young stellar objects, adding to the growing evidence that disk-mediated accretion is a phenomenon also occurring in high-mass stars, the formation mechanism of which is still poorly understood. Of the observed jets from massive young stars, none is in the optical regime (due to these being typically highly obscured by their native material), and none are found outside of the Milky Way. Here, we report the detection of HH 1177, the first extragalactic optical ionized jet originating from a massive young stellar object located in the Large Magellanic Cloud. The jet is highly collimated over the entire measured extent of at least 10 pc, and has a bipolar geometry. The presence of a jet indicates ongoing, disk-mediated accretion, and together with the high degree of collimation, this system is therefore likely to be an up-scaled version of low-mass star formation. We conclude that the physics governing jet launching and collimation is independent of stellar mass.
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We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO_2 ice at 15.2 micron and the ice band between 5 and 7 micron that includes contributions from the bending mode of water ice at 6 micron amongst other ice species. The 5-7 micron band is difficult to identify in our LMC sample due to the conspicuous presence of PAH emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8 micron ice feature attributed to ammonium and methanol. We model the CO_2 band in detail, using the combination of laboratory ice profiles available in the literature. We find that a significant fraction (> 50%) of CO_2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO_2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO_2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC.
Studies of young stellar objects (YSOs) in the Galaxy have found that a significant fraction exhibit photometric variability. However, no systematic investigation has been conducted on the variability of extragalactic YSOs. Here we present the first variability study of massive YSOs in a $sim 1.5,mathrm{deg^2}$ region of the Large Magellanic Cloud (LMC). The aim is to investigate whether the different environmental conditions in the metal-poor LMC ($sim$ 0.4-0.5 Z_sun) have an impact on the variability characteristics. Multi-epoch near-infrared (NIR) photometry was obtained from the VISTA Survey of the Magellanic Clouds (VMC) and our own monitoring campaign using the VISTA telescope. By applying a reduced $chi^2$-analysis, stellar variability was identified. We found 3062 candidate variable stars from a population of 362 425 stars detected. Based on several Spitzer studies, we compiled a sample of high-reliability massive YSOs: a total of 173 massive YSOs have NIR counterparts ($K_{mathrm{s}}sim 18.5,$mag) in the VMC catalogue, of which 39 display significant ($>3sigma$) variability. They have been classified as eruptive, fader, dipper, short-term variable and long-period variable YSOs based mostly on the appearance of their $K_{mathrm{s}}$ band light curves. The majority of YSOs are aperiodic, only five YSOs exhibit periodic lightcurves. The observed amplitudes are comparable or smaller than those for Galactic YSOs (only two Magellanic YSOs exhibit $Delta K_{mathrm{s}}>1,$mag), not what would have been expected from the typically larger mass accretion rates observed in the Magellanic Clouds.
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-IR (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 the $H$ and $K$ band are rich in emission lines that are mainly associated with ejection and accretion processes. Spectro-astrometry is applied to the Br$gamma$ line, and for the first time, to the Br$alpha$ 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 ($sim216^{circ}$). ISAAC spectroscopy reveals H$_2$ 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 $dot{M}_mathrm{ejec}sim10^{-4}mathrm{,M_odot,yr^{-1}}$ and the thrust is $dot{P}sim10^{-2}mathrm{,M_odot,yr^{-1},km,s^{-1}}$ , 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 $lesssim8%$ 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.
Highly-collimated gas ejections are among the most dramatic structures in the Universe, observed to emerge from very different astrophysical systems - from active galactic nuclei down to young brown dwarf stars. Even with the huge span in spatial scales, there is convincing evidence that the physics at the origin of the phenomenon, namely the acceleration and collimation mechanisms, is the same in all classes of jets. Here we report on the discovery of a giant, highly-collimated jet from Sanduleaks star in the Large Magellanic Cloud (LMC). With a physical extent of 14 parsecs at the distance of the LMC, it represents the largest stellar jet ever discovered, and the first resolved stellar jet beyond the Milky Way. The kinematics and extreme chemical composition of the ejecta from Sanduleaks star bear strong resemblance with the low-velocity remnants of SN1987A and with the outer filaments of the most famous supernova progenitor candidate, i.e., eta Carinae. Moreover, the precise knowledge of the jets distance implies that it will be possible to derive accurate estimates of most of its physical properties. Sanduleaks bipolar outflow will thus become a crucial test-bed for future theoretical modeling of astrophysical jets.
The process of massive star ($Mgeq8~M_odot$) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud (LMC). We took advantage of the spectral resolution and wavelength coverage of X-shooter (300-2500 nm), mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect H$alpha,beta$, [O I] 630.0 nm, Ca II infrared triplet, [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H$_2$ 2121.8 nm, Br$gamma$, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of 10 candidates as bona fide MYSOs. We compare our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We compute lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Br$gamma$ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features such as fluorescent Fe II and CO bandheads which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features.
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