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Register a new userSearch for High-Mass Protostellar Objects in Cold IRAS Sources
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We present the results of CS J=2-1 mapping observations towards 39 massive star-forming regions selected from the previous CO line survey of cold IRAS sources with high-velocity CO flows along the Galactic plane (Yang et al. 2002). All sources are detected in CS J=2-1 showing the existence of CS clumps around the IRAS sources. However, one-third of the sources are not deeply embedded in the dense clumps by comparison of the central powering IRAS sources and the morphologies of CS clumps. Physical parameters of the dense molecular clumps are presented. We have identified 12 high-mass protostellar object (HMPO) candidates by checking the association between the dense cores and the IRAS sources, the detection of water maser, and the radio properties towards the IRAS sources. We find that the HMPO sources are characterized by low FIR luminosity to virial mass ratios since they are in very early evolutionary stages when the massive protostars have not reached their full luminosities, which are typical for zero-age main sequence stars. Large turbulent motion in the HMPO sources may be largely due to the large kinetic energy ejected by the central protostars formed in the dense clumps. However, alternative means or undetected outflows may also be responsible for the turbulence in the clumps.
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The role of accretion disks in the formation of low-mass stars has been well assessed by means of high angular resolution observations at various wavelengths. These findings confirm the prediction that conservation of angular momentum during the collapse leading to the formation of a star is bound to produce flattening and rotation of the collapsing core. What about high-mass stars? At present, several authors have reported on detections of disks around high-mass YSOs. Notwithstanding these important results, the presence of disks rotating about high-mass stars is not sufficient by itself to prove unambiguously the accretion model: what is needed is iron-clad evidence of infall. Such evidence is very difficult to find, as the free-fall velocity becomes significant only very close to the accreting star, i.e., over a region of a few 0.01 pc ($sim$2000 au), which is very difficult to access and disentangle from the surrounding quiescent or rotating material. In this chapter we discuss how to characterize the infall of material in a sample of 36 high-mass accretion disk candidates covering a broad range of luminosities, from 10$^3$ $L_odot$ to 10$^6$ $L_odot$, compiled by Beltran & de Wit (2016) with the next generation Very Large Array (ngVLA).
We are carrying out multi-frequency radio continuum observations, using the Australia Telescope Compact Array, to systematically search for collimated ionized jets towards high-mass young stellar objects (HMYSOs). Here we report observations at 1.4, 2.4, 4.8 and 8.6 GHz, made with angular resolutions of about 7, 4, 2, and 1 arcsec, respectively, towards six objects of a sample of 33 southern HMYSOs thought to be in very early stages of evolution. The objects in the sample were selected from radio and infrared catalogs by having positive radio spectral indices and being luminous (L_bol > 20,000 L_sun), but underluminous in radio emission compared to that expected from its bolometric luminosity. This criteria makes the radio sources good candidates for being ionized jets. As part of this systematic search, two ionized jets have been discovered: one previously published and the other reported here. The rest of the observed candidates correspond to three hypercompact hii regions and two ultracompact hii regions. The two jets discovered are associated with two of the most luminous (70,000 and 100,000 Lsun) HMYSOs known to harbor this type of objects, showing that the phenomena of collimated ionized winds appears in the formation process of stars at least up to masses of ~ 20 M_sun and provides strong evidence for a disk-mediated accretion scenario for the formation of high-mass stars. From the incidence of jets in our sample, we estimate that the jet phase in high-mass protostars lasts for 40,000 yr.
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.
We have analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) cycle 2 data of band 6 toward the G345.4938+01.4677 massive young protostellar object (G345.5+1.47 MYSO) in the IRAS 16562--3959 high-mass star-forming region with an angular resolution of $sim 0.3$, corresponding to $sim 760$ au. We spatially resolve the central region which consists of three prominent molecular emission cores. A hypercompact (HC) H$_{rm {II}}$ region (Core A) and two molecule-rich cores (Core B and Core C) are identified using the moment zero images of the H30$alpha$ line and a CH$_{3}$OH line, respectively. Various oxygen-bearing complex organic molecules (COMs), such as (CH$_{3}$)$_{2}$CO and CH$_{3}$OCHO, have been detected toward the positions of Core B and Core C, while nitrogen-bearing species, CH$_{3}$CN, HC$_{3}$N and its $^{13}$C isotopologues, have been detected toward all of the cores. We discuss the formation mechanisms of H$_{2}$CO by comparing the spatial distribution of C$^{18}$O with that of H$_{2}$CO. The $^{33}$SO emission, on the other hand, shows a ring-like structure surrounding Core A, and it peaks on the outer edge of the H30$alpha$ emission region. These results imply that SO is enhanced in a shock produced by the expanding motion of the ionized region.
Context. Magnetic fields can affect significantly the star formation process. The theory of the magnetically-driven collapse in a uniform field predicts that initially the contraction happens along the field lines. When the gravitational pull grows strong enough, the magnetic field lines pinch inwards, giving rise to a characteristic hourglass shape. Aims. We investigate the magnetic field structure of a young Class 0 object, IRAS 15398-3359, embedded in the Lupus I cloud. Previous observations at large scales suggest that this source evolved in an highly magnetised environment. This object thus appears an ideal candidate to study the magnetically driven core collapse in the low-mass regime. Methods. We have performed polarisation observations of IRAS 15398-3359 at 214$mu$m using the SOFIA/HAWC+ instrument, thus tracing the linearly polarised thermal emission of cold dust. Results. Our data unveil a significant bend of the magnetic field lines due to the gravitational pull. The magnetic field appears ordered and aligned with the large-scale B-field of the cloud and with the outflow direction. We estimate a magnetic field strength of $B= 78 mu$G, expected to be accurate within a factor of two. The measured mass-to-flux parameter is $lambda= 0.95$, indicating that the core is in a transcritical regime.
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