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ISM, star formation and astrochemistry

The paradigm for low-mass star formation posits that stars form from the gravitational collapse of dense cores. As the material infalls onto the central protostar, a circumstellar disk forms that serves as a conduit to accrete material onto the protostar. Bipolar outflows from protostars may play a significant role in the evolution of stars by disrupting the surrounding cloud. Outflows can also remove angular momentum for the protostellar system, which allows the protostar to continue to accrete material from the disk.

ALMA plays a critical role in testing and refining the star formation paradigm. Besides by observing all phases in the formation of low-mass stars, ALMA has the sensitivity and angular resolution to probe if the paradigm extends to high mass stars and stellar clusters.


Low mass star formation

Despite being one of the central tenets in the star formation paradigm, unambiguous evidence for infalling material onto a central protostar has been elusive given the complex velocity, density, and temperature of the gas and dust in dense cores. ALMA spectral observations of the globule B335 has provided the strong observational evidence for infalling material by detecting redshifted absorption in spectral lines against the continuum source (see Figure 3.1).


Figure 3.1: Left: ALMA 345 GHz continuum image of the protostar B335. The green ellipse in the lower left corner shows the beam size. Right: Observed (black) and model (red) spectra for HCO+ and HCN J=4-3 toward the continuum source (left panels) and offset from the continuum source (right panels). The vertical line shows the systematic velocity of the protostar. While redshifted “self-absorption” is observed in each of the spectra, redshifted absorption below the continuum is only detected toward protostar, which is the spectral signature of infalling material. Figures from Evans et al. (2016).


ALMA has also produced spectacular maps of the bipolar outflows from young protostars. Figure 3.2 shows an image of the bipolar outflow from a protostar in the Serpens molecular cloud. The outflow contains distinct knots in the molecular emission that suggests new material is ejected in the outflow every ~ 300 years.

A long standing issue is whether these outflows are launched close to the star or over the entire disk. Figure 3.3 shows a high resolution image of the outflow from the protostar TMC1A. The image shows that in this protostar, the outflow originates over a 25 radius region, suggesting a disk origin for the outflow.


Figure 3.2: ALMA image of the CO J=2-1 emissions from the molecular outflow from a protostar in the Serpens South molecular cloud (Plunkett et al. 2015). The outflow lobes show distinct peaks that may represent episodic eject from the central protostar. Credit: B. Saxton (NRAO/AUI/NSF); A. Plunkett et al.; ALMA (NRAO/ESO/NAOJ).



Figure 3.3: Annotated three-color image of the CO J=2-1 emission from protostar TMC1A. The blueshifted and redshifted CO emission is shown by red and blue respectively, and the continuum emission from the protostar is shown in green. The blueshifted emission is observed to originate from a region extending out to 25 AU in radius, indicating the launch region for the outflow in TMC1A is from a disk wind. Figure from Bjerkeli et al. (2016).


Approximately half of solar-type stars are members of multiple systems. The likely formation mechanisms are either gravitational instabilities within a disk or large scale fragmentation of dense cores. ALMA continuum observations of the triple system L1448 (see Figure 3.4) has captured the formation of triple system forming with a disk connected by a spiral arm. The disk is inferred to be gravitationally unstable, consistent with models where multiple star systems form through disk gravitational instabilities. 


Figure 3.4: ALMA 1.3mm continuum image of the triple system L1448 IRS3B. The crosses indicate the position of the three protostars (IRS3B-a, b, and c). The three protostars are embedded within a spiral arm that appears to emerge from the close pair (IRS3B-a and b) and extends to IRS3B-c. Figure from Tobin et al. (2006).


Formation of high mass stars and clusters

One of the outstanding questions in star formation is to what extent the paradigm for low-mass star formation can be extended to high-mass stars, and in particular, if high mass stars are surrounded by circumstellar disks. ALMA observations of the molecular line emission toward young O-type star AFGL 4176 shows a velocity gradient of the gas surrounding the star that is consistent with the presence of the circumstellar disks (see Figure 3.5).


Figure 3.5: ALMA continuum emission (contours) superimposed on the velocity map of the CH3CN J=13-12, K=3 emission around the high mass protostar AFGL 4176. The gradient in the CH3CN emission is consistent with a Keplerian rotation of a disk around an O-type star. Figure from Johnston et al. (2015).


ALMA high resolution observations of dense, compact clumps of gas are revealing the initial conditions of stellar clusters. Figure 3.6 presents dust continuum and spectral line images of molecular clump G0.253+0.016, which is located within ~ 100 pc of the Galactic center and and is thought to be the progenitor of a massive cluster since it contains ~ 105 Msolar of gas and dust within a radius of 2.9 pc. The images of the different molecular tracers exhibit quite different morphologies, reflecting the variations in the excitation and chemical conditions within the clump. In contrast to stellar clusters, G0.253+0.016 clump does not contain any central high-mass concentration, suggesting that if the clump will form a cluster, it remains to evolve into a centrally evolved state.


Figure 3.6: ALMA continuum and molecular line images of the G0.253+0.016 cloud clump, also known as the “Brick”. Single dish spacings were provided by the Mopra 22 m single dish telescope for the spectral line images and Herschel for the dust continuum. Figure from Rathborne et al. (2015).


Magnetic fields are thought to play a critical role in the evolution of molecular clouds and the formation of stars. ALMA polarimetric continuum observations of the W43 high-mass star forming region show a fragmented filament threaded by an ordered magnetic field (see Figure 3.7). The inferred ratios of the fragment masses to the magnetic field ratio are super-critical (>> 1), suggesting that the magnetic field is dominated by gravity at this stage of evolution in W43-MM1.


Figure 3.7: Magnetic morphology in the W43-MM1 high-mass star forming region. The color scale shows Stokes I emission in the 1 mm continuum observed with ALMA. The vectors indicate the magnetic field morphology. Figure from Cortes et al. (2016).

Molecular lines probed by ALMA are diagnostic of the physical conditions and structure in the interstellar medium. ALMA observations of CO and HCO+ at 1 arcsec of the Orion Nebula have resolved the transition from atomic to molecular gas across the Orion Bar (see Figure 3.8). The observations reveal a highly fragmented ridge that may represent photoablative gas flows and instabilities at the molecular cloud surface. These observations also demonstrates the importance dynamical and non-equilibrium effects on the cloud evolution.


Figure 3.8: Left: Three color composite of the Orion molecular cloud, where the HCO+ J=3-2 emission from ALMA and IRAM single dish is shown in red, hot ionized gas traced by [SII] is in green, and neutral atomic gas traced by [OI] is in blue. Right: Close-up of the Orion Bar region. Figure from Goicoechea et al. (2016).


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