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Register a new userA Substellar-Mass Protostar and its Outflow of IRAS 15398-3359 Revealed by Subarcsecond-Resolution Observations of H$_2$CO and CCH
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Sub-arcsecond ($0.^{primeprime}5$) images of H$_2$CO and CCH line emission have been obtained in the $0.8$ mm band toward the low-mass protostar IRAS 15398-3359 in the Lupus 1 cloud as one of the Cycle 0 projects of the Atacama Large Millimeter/Submillimeter Array. We have detected a compact component concentrated in the vicinity of the protostar and a well-collimated outflow cavity extending along the northeast-southwest axis. The inclination angle of the outflow is found to be about $20^circ$, or almost edge-on, based on the kinematic structure of the outflow cavity. This is in contrast to previous suggestions of a more pole-on geometry. The centrally concentrated component is interpreted by use of a model of the infalling rotating envelope with the estimated inclination angle, and the mass of the protostar is estimated to be less than $0.09 M_odot$. Higher spatial resolution data are needed to infer the presence of a rotationally supported disk for this source, hinted at by a weak high-velocity H$_2$CO emission associated with the protostar.
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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.
We have observed the very low-mass Class 0 protostar IRAS 15398-3359 at scales ranging from 50 au to 1800 au, as part of the ALMA Large Program FAUST. We uncover a linear feature, visible in H2CO, SO, and C18O line emission, which extends from the source along a direction almost perpendicular to the known active outflow. Molecular line emission from H2CO, SO, SiO, and CH3OH further reveals an arc-like structure connected to the outer end of the linear feature and separated from the protostar, IRAS 15398-3359, by 1200 au. The arc-like structure is blue-shifted with respect to the systemic velocity. A velocity gradient of 1.2 km/s over 1200 au along the linear feature seen in the H2CO emission connects the protostar and the arc-like structure kinematically. SO, SiO, and CH3OH are known to trace shocks, and we interpret the arc-like structure as a relic shock region produced by an outflow previously launched by IRAS 15398-3359. The velocity gradient along the linear structure can be explained as relic outflow motion. The origins of the newly observed arc-like structure and extended linear feature are discussed in relation to turbulent motions within the protostellar core and episodic accretion events during the earliest stage of protostellar evolution.
Subarcsecond-resolution images of the rotational line emissions of CS and c-C$_3$H$_2$ obtained toward the low-mass protostar IRAS 04368$+$2557 in L1527 with the Atacama Large Millimeter/submillimeter Array are investigated to constrain the orientation of the outflow/envelope system. The distribution of CS consists of an envelope component extending from north to south and a faint butterfly-shaped outflow component. The kinematic structure of the envelope is well reproduced by a simple ballistic model of an infalling rotating envelope. Although the envelope has a nearly edge-on configuration, the inclination angle of the rotation axis from the plane of the sky is found to be 5$^circ$, where we find that the western side of the envelope faces the observer. This configuration is opposite to the direction of the large-scale ($sim$ 10$^4$ AU) outflow suggested previously from the $^{12}$CO ($J$=3$-$2) observation, and to the morphology of infrared reflection near the protostar ($sim$ 200 AU). The latter discrepancy could originate from high extinction by the outflow cavity of the western side, these discrepancies or may indicate that the outflow axis is not parallel to the rotation axis of the envelope. Position-velocity diagrams show the accelerated outflow cavity wall, and its kinematic structure in the 2000 AU scale is explained by a standard parabolic model with the inclination angle derived from the analysis of the envelope. The different orientation of the outflow between the small and large scale implies a possibility of precession of the outflow axis. The shape and the velocity of the outflow in the vicinity of the protostar are compared with those of other protostars.
Sub-arcsecond images of the rotational line emission of CS and SO have been obtained toward the Class I protostar IRAS 04365$+$2535 in TMC-1A with ALMA. A compact component around the protostar is clearly detected in the CS and SO emission. The velocity structure of the compact component of CS reveals infalling-rotating motion conserving the angular momentum. It is well explained by a ballistic model of an infalling-rotating envelope with the radius of the centrifugal barrier (a half of the centrifugal radius) of 50 AU, although the distribution of the infalling gas is asymmetric around the protostar. The distribution of SO is mostly concentrated around the radius of the centrifugal barrier of the simple model. Thus a drastic change in chemical composition of the gas infalling onto the protostar is found to occur at a 50 AU scale probably due to accretion shocks, demonstrating that the infalling material is significantly processed before being delivered into the disk.
We present near infrared spectroscopic observations of 19 molecular clouds made using the AKARI satellite, and the data reduction pipeline written to analyse those observations. The 2.5 --~5 $mu$m spectra of 30 objects -- 22 field stars behind quiescent molecular clouds and eight low mass YSOs in cores -- were successfully extracted using the pipeline. Those spectra are further analysed to calculate the column densities of key solid phase molecular species, including H$_2$O, CO$_2$, CO, and OCN$^-$. The profile of the H$_2$O ice band is seen to vary across the objects observed and we suggest that the extended red wing may be an evolutionary indicator of both dust and ice mantle properties. The observation of 22 spectra with fluxes as low as $<$~5 mJy towards background stars, including 15 where the column densities of H$_2$O, CO and CO$_2$ were calculated, provides valuable data that could help to benchmark the initial conditions in star-forming regions prior to the onset of star formation.
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