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Register a new userMeasuring the ionisation fraction in a jet from a massive protostar
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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 $sim5-12%$ 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 $sim10$ solar masses.
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We present the first resolved observations of the 1.3mm polarized emission from the disk-like structure surrounding the high-mass protostar Cepheus A HW2. These CARMA data partially resolve the dust polarization, suggesting an uniform morphology of polarization vectors with an average position angle of 57 degrees and an average polarization fraction of 2.0%. The distribution of the polarization vectors can be attributed to (1) the direct emission of magnetically aligned grains of dust by a uniform magnetic field, or (2) the pattern produced by the scattering of an inclined disk. We show that both models can explain the observations, and perhaps a combination of the two mechanisms produce the polarized emission. A third model including a toroidal magnetic field does not match the observations. Assuming scattering is the polarization mechanism, these observations suggest that during the first few 10000 years of high-mass star formation, grain sizes can grow from 1 to several 10s micron.
We report the discovery of a massive protostar M17 MIR embedded in a hot molecular core in M17. The multi-wavelength data of M17 MIR during 1993 to 2019 show significant mid-IR (MIR) variations, which can be split into three stages, the decreasing phase during 1993.03 to mid 2004, the quiescent phase during mid 2004 to mid 2010, and the re-brightening phase since mid 2010 untill now. The H2O maser emission variation toward M17 MIR, together with the MIR variation, indicate an enhanced disk accretion rate onto M17 MIR during the decreasing and re-brightening phase. According to the kinematics of H2O maser spots, accretion rate ~7x10^-4 Msun/yr is estimated in the initial stage of the re-brightening phase, and a higher rate ~2x10^-3 Msun/yr is obtained in later stage, given by the MIR flux increased by a factor of 3. Radiative transfer modeling of SEDs of M17~MIR in the 2005 (quiescent) and 2017 epoch (accretion outburst) constrains the basic stellar parameters of M17 MIR, which is an intermediate-mass protostar (M~5.4 Msun) with lower accretion rate ~1.1x10^-5 Msun in quiescent and two orders of magnitude higher rate ~1.7x10^-3 Msun/yr in outburst. The enhanced accretion rate during outburst induces the luminosity outburst $Delta Lapprox7600 $Lsun, and a larger stellar radius is required to produce accretion rate consistent with observations. The decreasing and re-brightening phase reflect two accretion bursts ($Delta tsim 9-20$ yr) with burst magnitudes of 2 mag, separated by a quiescent phase lasting $sim6$ yr. The fraction time in accretion ourbusrt is about 83% over 26 yr. M17 MIR is the youngest one among the six confirmed sources with accretion burst. The extreme youth of M17 MIR suggests that minor accretion bursts are frequent at the earliest stages of massive star formation.
We present SMA CO(2-1) observations toward the protostellar jet driven by SVS13A, a variable protostar in the NGC1333 star-forming region. The SMA CO(2-1) images show an extremely high-velocity jet composed of a series of molecular bullets. Based on the SMA CO observations, we discover clear and large systematic velocity gradients, perpendicular to the jet axis, in the blueshifted and redshifted bullets. After discussing several alternative interpretations, such as twin-jets, jet precession, warped disk, and internal helical shock, we suggest that the systematic velocity gradients observed in the bullets result from the rotation of the SVS13A jet. From the SMA CO images, the measured rotation velocities are 11.7-13.7 km/s for the blueshifted bullet and 4.7+/-0.5 km/s for the redshifted bullet. The estimated specific angular momenta of the two bullets are comparable to those of dense cores, about 10 times larger than those of protostellar envelopes, and about 20 times larger than those of circumstellar disks. If the velocity gradients are due to the rotation of the SVS13A jet, the significant amount of specific angular momenta of the bullets indicates that the rotation of jets/outflows is a key mechanism to resolve the so-called angular momentum problem in the field of star formation. The kinematics of the bullets suggests that the jet launching footprint on the disk has a radius of about 7.2-7.7 au, which appears to support the extended disk-wind model. We note that further observations are needed to comprehensively understand the kinematics of the SVS13A jet, in order to confirm the rotation nature of the bullets.
We report the results of VERA multi-epoch VLBI 22 GHz water maser observations of S255IR-SMA1, a massive young stellar object located in the S255 star forming region. By annual parallax the source distance was measured as D = 1.78 +-0.12 kpc and the source systemic motion was (u alpha cos d, u d) = (-0.13 +- 0.20, -0.06 +- 0.27) mas yr-1. Masers appear to trace a U-shaped bow shock whose morphology and proper motions are well reproduced by a jet-driven outflow model with a jet radius of about 6 AU. The maser data, in the context of other works in the literature, reveal ejections from S255IR-SMA1 to be episodic, operating on timescales of ~1000 years.
We used ALMA to observe the star-forming region GGD27 at 1.14 mm with an unprecedented angular resolution, 40 mas (56 au) and sensitivity (0.002 Msun). We detected a cluster of 25 continuum sources, most of which are likely tracing disks around Class 0/I protostars. Excluding the two most massive objects, disks masses are in the range 0.003-0.05 Msun. The analysis of the cluster properties indicates that GGD27 displays moderate subclustering. This result combined with the dynamical timescale of the radio jet (10000 years) suggests the youthfulness of the cluster. The lack of disk mass segregation signatures may support this too. We found a clear paucity of disks with Rdisk >100 au. The median value of the radius is 34 au, smaller than the median of 92 au for Taurus but comparable to the value found in Ophiuchus and in the Orion Nebula Cluster. In GGD27 there is no evidence of a distance-dependent disk mass distribution (i. e., disk mass depletion due to external photoevaporation), most likely due to the cluster youth. There is a clear deficit of disks for distances <0.02 pc. Only for distances >0.04 pc stars can form larger and more massive disks, suggesting that dynamical interactions far from the cluster center are weaker, although the small disks found could be the result of disk truncation. This work demonstrates the potential to characterize disks from low-mass YSOs in distant and massive (still deeply embedded) clustered environments.
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