LOW-MASS STAR FORMATION: CURRENT STATUS AND FUTURE PROGRESS WITH ALMA

Abstract

Low-mass star-formation studies deal with the birth of individual solar-type stars as it occurs in nearby molecular clouds. While this isolated mode of star formation may not represent the most common form of stellar birth, its study often provides first evidence for the general ingredients of star formation, such as gravitational infall, disk formation, or outflow acceleration. Here I briefly review the current status and the main challenges in our understanding of low-mass star formation, with emphasis in the still mysterious pre-stellar phase. In addition to presenting by-now classical work, I also show how ALMA is starting to play a decisive role driving progress in this field.

Figure 1. Evidence for bers in the L1495/B213 lament of Taurus. The background grey-scale image represents the dust continuum emission as mapped by Palmeirim et al. (2013). The colored lines mark the location of the backbones of the bers as determined by Hacar et al. (2013) from the analysis of C¹⁸O spectra. Note how what appears as a single large-scale lament is in fact a network of intertwined bers.

Figure 2. Schematic illustration of the fray and fragment scenario of core formation. The large scale lament is formed by the collision of two ows (left). Turbulent fragmentation gives rise to multiple velocity-coherent bers (middle). Some bers become massive enough to fragment gravitationally and produce multiple dense cores (right) From Tafalla & Hacar (2015).

Figure 3. Evidence for bers in the Orion Integral Shape Filament. Left: SCUBA 850 m continuum image from Johnstone & Bally (1999). For reference, blue triangles represent Spitzer protostars, blue crosses represent continuum sources, white stars represent the Trapezium sources, and a yellow star represents the Orion BN source. Middle: ALMA plus IRAM 30m N₂H⁺(1-0) image from Hacar et al. (2018) showing evidence for multiple ber-like structures. Right: Location of the bers as determined from the analysis of the N₂H⁺(1-0) velocity structure. From Hacar et al. (2018).

Figure 4. Chain of dense cores in the B213 lament of Taurus as mapped in N₂H⁺(1-0) by Tafalla & Hacar (2015). Note how the core arrangement maintains the elongated structure of the original ber, suggesting that core formation has occurred by the gravitational fragmentation of the gas in the ber.

Figure 5. Evidence for chemical dierentiation in the L1517B dense core. The top panels present maps of the 1.2mm continuum, N₂H⁺(1-0), and C¹⁸O(1-0) emission. The bottom panels present the corresponding radial proles of emission (black squares) together with radiative transfer ts. For N₂H⁺(1-0), a constant abundance model ts the data, while for C¹⁸O(1-0), a constant abundance model (red line) only ts the outer emission. To t the data, the model requires an order of magnitude drop in the central abundance (blue line). From Tafalla et al. (2004).

Figure 6. Channel maps of the CO(2-1) extremely high velocity (EHV) emission from two elds along the IRAS 04166 jet, as mapped with ALMA by Tafalla et al. (2017). The northern eld corresponds to jet knot B6 and the southern eld to jet knot R6 in the notation of Santiago-Garca et al. (2009). The velocity of each channel has been corrected for the jet velocity, and is indicated in each panel. Note how in both elds the emission moves from southwest to northeast with increasing velocity, and it delineates an elliptical region. This pattern indicates that the gas in each jet knot lies in a disk-like structure that expands away from the jet axis, as expected if the gas is being laterally ejected in an internal jet shock. See Tafalla et al. (2017) for a full discussion.