• Received : 2018.08.06
  • Accepted : 2018.12.07
  • Published : 2018.12.31


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.

CMHHCI_2018_v33n3_45_f0001.png 이미지

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.

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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).

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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).

CMHHCI_2018_v33n3_45_f0004.png 이미지

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.

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Figure 5. Evidence for chemical di erentiation 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).

CMHHCI_2018_v33n3_45_f0006.png 이미지

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.


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  1. Aikawa, Y., Herbst, E., Roberts, H., & Caselli, P., 2005, Molecular Evolution in Collapsing Prestellar Cores. III. Contraction of a Bonnor-Ebert Sphere, ApJ, 620, 330
  2. Alves, J. F., Lada, C. J., & Lada, E. A., 2001, Internal Structure of a Cold Dark Molecular Cloud Inferred from the Extinction of Background Starlight, Nature, 409, 159
  3. Alves, J., Lombardi, M., & Lada, C. J., 2007, The Mass Function of Dense Molecular Cores and the Origin of the IMF, A&A, 462, L17
  4. Andre, P., Men'shchikov, A., Bontemps, S., et al., 2010, From Filamentary Clouds to Prestellar Cores to the Stellar IMF: Initial Highlights from the Herschel Gould Belt Survey, A&A, 518, L102
  5. Andre, P., Di Francesco, J., Ward-Thompson, D., et al., 2014, From Filamentary Networks to Dense Cores in Molecular Clouds: Toward a New Paradigm for Star Formation, Protostars and Planets VI, 27
  6. Arce, H. G., Mardones, D., Corder, S. A., et al., 2013, ALMA Observations of the HH 46/47 Molecular Outflow, ApJ, 774, 39
  7. Arzoumanian, D., Andre, P., Didelon, P., et al., 2011, Characterizing Interstellar Filaments with Herschel in IC 5146, A&A, 529, L6
  8. Bacmann, A., Lefloch, B., Ceccarelli, C., et al., 2003, CO Depletion and Deuterium Fractionation in Prestellar Cores, ApJL, 585, L55
  9. Beichman, C. A., Myers, P. C., Emerson, J. P., et al., 1986, Candidate Solar-Type Protostars in nearby Molecular Cloud Cores, ApJ, 307, 337
  10. Benson, P. J. & Myers, P. C., 1989, A Survey for Dense Cores in Dark Clouds, ApJSS, 71, 89
  11. Benson, P. J., Caselli, P., & Myers, P. C., 1998, Dense Cores in Dark Clouds. XI. A Survey for $N_2H^+$, $C_3H_2$, and CCS, ApJ, 506, 743
  12. Bergin, E. A., Alves, J., Huard, T., & Lada, C. J., 2002, $N_2H^+$ and $C^{18}O$ Depletion in a Cold Dark Cloud, ApJL, 570, L101
  13. Bergin, E. A. & Langer, W. D., 1997, Chemical Evolution in Preprotostellar and Protostellar Cores, ApJ, 486, 316
  14. Bergin, E. A. & Tafalla, M., 2007, Cold Dark Clouds: The Initial Conditions for Star Formation, ARAA, 45, 339
  15. Bisschop, S. E., Fraser, H. J., Oberg, K. I., van Dishoeck, E. F., & Schlemmer, S., 2006, Desorption Rates and Sticking Coefficients for CO and $N_2$ Interstellar Ices, A&A, 449, 1297
  16. Bonnor, W. B., 1956, Boyle's Law and Gravitational Instability, MNRAS, 116, 351
  17. Cabrit, S., Goldsmith, P. F., & Snell, R. L., 1988, Identification of RNO 43 and B335 as Two Highly Collimated Bipolar Flows Oriented nearly in the Plane of the Sky, ApJ, 334, 196
  18. Caselli, P., Walmsley, C. M., Tafalla, M., Dore, L., & Myers, P. C., 1999, CO Depletion in the Starless Cloud Core L1544, ApJL, 523, L165
  19. Caselli, P., Benson, P. J., Myers, P. C., & Tafalla, M., 2002, Dense Cores in Dark Clouds. XIV. $N_2H^+$ (1-0) Maps of Dense Cloud Cores, ApJ, 572, 238
  20. Caselli, P., Vastel, C., Ceccarelli, C., et al., 2008, Survey of ortho-$H_2D^+(1_{1,0}-1_{1,1})$ in dense cloud cores, A&A, 492, 703
  21. Caselli, P., Keto, E., Bergin, E. A., et al., 2012, First Detection of Water Vapor in a Pre-stellar Core, ApJL, 759, L37
  22. Ceccarelli, C., Caselli, P., Herbst, E., Tielens, A. G. G. M., & Caux, E. 2007, Extreme Deuteration and Hot Corinos: The Earliest Chemical Signatures of Low-Mass Star Formation, Protostars and Planets V, 47
  23. Chandler, C. J., Brogan, C. L., Shirley, Y. L., & Loinard, L. 2005, IRAS 16293-2422: Proper Motions, Jet Precession, the Hot Core, and the Unambiguous Detection of Infall, ApJ, 632, 371
  24. Codella, C., Cabrit, S., Gueth, F., et al., 2014, The ALMA View of the Protostellar System HH212. The Wind, the Cavity, and the Disk, A&A, 568, L5
  25. Crapsi, A., Caselli, P., Walmsley, C. M., et al., 2005, Probing the Evolutionary Status of Starless Cores through $N_2H^+$ and $N_2D^+$ Observations, ApJ, 619, 379
  26. Crapsi, A., Caselli, P., Walmsley, M. C., & Tafalla, M., 2007, Observing the Gas Temperature Drop in the High-density Nucleus of L 1544 , A&A, 470, 221
  27. Duchene, G. & Kraus, A., 2013, Stellar Multiplicity, ARAA, 51, 269
  28. Dunham, M. M., Offner, S. S. R., Pineda, J. E., et al., 2016, An ALMA Search for Substructure, Fragmentation, and Hidden Protostars in Starless Cores in Chamaeleon I, ApJ, 823, 160
  29. Ebert, R., 1955, Uber die Verdichtung von H I-Gebieten. Zeitschrift fur Astrophysik, 37, 217
  30. Evans, N. J., II, 1999, Physical Conditions in Regions of Star Formation , ARAA, 37, 311
  31. Evans, N. J., II, Rawlings, J. M. C., Shirley, Y. L., & Mundy, L. G., 2001, Tracing the Mass during Low-Mass Star Formation. II. Modeling the Submillimeter Emission from Preprotostellar Cores, ApJ, 557, 193
  32. Evans, N. J., II, Di Francesco, J., Lee, J. -E., et al., 2015, Detection of Infall in the Protostar B335 with ALMA, ApJ, 814, 22
  33. Fayolle, E. C., Balfe, J., Loomis, R., et al., 2016, $N_2$ and CO Desorption Energies from Water Ice, ApJL, 816, L28
  34. Forbrich, J., Oberg, K., Lada, C. J., et al., 2014, Some like it cold: molecular emission and effective dust temperatures of dense cores in the Pipe Nebula, A&A, 568, A27
  35. Frank, A., Ray, T. P., Cabrit, S., et al., 2014, Jets and Outflows from Star to Cloud: Observations Confront Theory, Protostars and Planets VI, 451
  36. Galli, D., Walmsley, M., & Goncalves, J., 2002, The structure and stability of molecular cloud cores in external radiation fields, A&A, 394, 275
  37. Girichidis, P., Federrath, C., Banerjee, R., & Klessen, R. S., 2011, Importance of the initial conditions for star formation - I. Cloud evolution and morphology, MNRAS, 413, 2741
  38. Goldsmith, P. F., 2001, Molecular Depletion and Thermal Balance in Dark Cloud Cores, ApJ, 557, 736
  39. Goldsmith, P. F., Heyer, M., Narayanan, G., et al., 2008, Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Linear Dynamic Range Spectral Line Mapping, ApJ, 680, 428
  40. Goodman, A. A., Benson, P. J., Fuller, G. A., & Myers, P. C., 1993, Dense cores in dark clouds. VIII - Velocity gradients, ApJ, 406, 528
  41. Hacar, A., Tafalla, M., Kauffmann, J., & Kovacs, A., 2013, Cores, Filaments, and Bundles: Hierarchical Core Formation in the L1495/B213 Taurus Region, A&A, 554, A55
  42. Hacar, A., Tafalla, M., & Alves, J., 2017, Fibers in the NGC 1333 Proto-Cluster, A&A, 606, A123
  43. Hacar, A., Tafalla, M., Forbrich, J., et al., 2018, An ALMA study of the Orion Integral Filament. I. Evidence for narrow fibers in a massive cloud, A&A, 610, A77
  44. Hartmann, L., 2002, Flows, Fragmentation, and Star Formation. I. Low-Mass Stars in Taurus, ApJ, 578, 914
  45. Henshaw, J. D., Caselli, P., Fontani, F., Jimenez-Serra, I., & Tan, J. C., 2014, The Dynamical Properties of Dense Filaments in the Infrared Dark Cloud G035.39-00.33, MNRAS, 440, 2860
  46. Henshaw, J. D., Jimenez-Serra, I., Longmore, S. N., et al., 2017, Unveiling the early-stage anatomy of a protocluster hub with ALMA, MNRAS, 464, L31
  47. Herczeg, G. J., Johnstone, D., Mairs, S., et al., 2017, How Do Stars Gain Their Mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, ApJ, 849, 43
  48. Johnstone, D. & Bally, J., 1999, JCMT/SCUBA Submillimeter Wavelength Imaging of the Integral-shaped Filament in Orion, ApJL, 510, L49
  49. Juvela, M., Demyk, K., Doi, Y., et al., 2015, Galactic cold cores. VI. Dust opacity spectral index, A&A, 584, A94
  50. Kandori, R., Nakajima, Y., Tamura, M., et al., 2005, Near-Infrared Imaging Survey of Bok Globules: Density Structure, AJ, 130, 2166
  51. Kirk, H., Dunham, M. M., Di Francesco, J., et al., 2017, ALMA Observations of Starless Core Substructure in Ophiuchus, ApJ, 838, 114
  52. Kuiper, T. B. H., Langer, W. D., & Velusamy, T., 1996, Evolutionary Status of the Pre-protostellar Core L1498, ApJ, 468, 761
  53. Lada, C. J., Muench, A. A., Rathborne, J., Alves, J. F., & Lombardi, M., 2008, The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure, ApJ, 672, 410
  54. Launhardt, R., Stutz, A. M., Schmiedeke, A., et al., 2013, The Earliest Phases of Star Formation (EPoS): a Herschel key project. The thermal structure of low-mass molecular cloud cores, A&A, 551, A98
  55. Lee, C. -F., Mundy, L. G., Reipurth, B., Ostriker, E. C., & Stone, J. M., 2000, CO Outflows from Young Stars: Confronting the Jet and Wind Models, ApJ, 542, 925
  56. Lee, C. -F., Hirano, N., Zhang, Q., et al., 2014, ALMA Results of the Pseudodisk, Rotating Disk, and Jet in the Continuum and $HCO^+$ in the Protostellar System HH 212, ApJ, 786, 114
  57. Lee, C. -F., Ho, P. T. P., Li, Z. -Y., et al., 2017, A rotating protostellar jet launched from the innermost disk of HH 212, Nature Astronomy, 1, 152
  58. Lee, C. -F., Li, Z. -Y., Codella, C., et al., 2018, A 100 au Wide Bipolar Rotating Shell Emanating from the HH 212 Protostellar Disk: A Disk Wind?, ApJ, 856, 14
  59. Lee, C. W., Myers, P. C., & Tafalla, M., 1999, A Survey of Infall Motions toward Starless Cores. I. CS (2-1) and $N_2H^+$ (1-0) Observations, ApJ, 526, 788
  60. Lee, C. W., Myers, P. C., & Tafalla, M., 2001, A Survey for Infall Motions toward Starless Cores. II. CS (2-1) and $N_2H^+$(1-0) Mapping Observations , ApJSS, 136, 703
  61. Lee, C. W., Myers, P. C., & Plume, R., 2004, A Survey for Infall Motions toward Starless Cores. III. CS (3-2) and $DCO^+$ (2-1) Observations , ApJSS, 153, 523
  62. Mardones, D., Myers, P. C., Tafalla, M., et al., 1997, A Search for Infall Motions toward Nearby Young Stellar Objects, ApJ, 489, 719
  63. Masson, C. R. & Chernin, L. M., 1993, Properties of Jetdriven Molecular Outflows, ApJ, 414, 230
  64. Maury, A. J., Girart, J. M., Zhang, Q., et al., 2018, Magnetically Regulated Collapse in the B335 Protostar? I. ALMA Observations of the Polarized Dust Emission, MNRAS, 477, 2760
  65. Megeath, S. T., Gutermuth, R., Muzerolle, J., et al., 2012, The Spitzer Space Telescope Survey of the Orion A and B Molecular Clouds. I. A Census of Dusty Young Stellar Objects and a Study of Their Mid-infrared Variability, AJ, 144, 192
  66. Molinari, S., Swinyard, B., Bally, J., et al. 2010, Clouds, Filaments, and Protostars: The Herschel Hi-GAL Milky Way, A&A, 518, L100
  67. Motte, F., Andre, P., & Neri, R., 1998, The Initial Conditions of Star Formation in the rho Ophiuchi Main Cloud: Wide-field Millimeter Continuum Mapping, A&A, 336, 150
  68. Myers, P. C., 1983, Dense Cores in Dark Clouds. III - Subsonic Turbulence, ApJ, 270, 105
  69. Myers, P. C., 2009, Filamentary Structure of Star-forming Complexes, ApJ, 700, 1609
  70. Nakamura, F., Takakuwa, S., & Kawabe, R., 2012, Substellar-mass Condensations in Prestellar Cores, ApJL, 758, L25
  71. Ohashi, N., Saigo, K., Aso, Y., et al., 2014, Formation of a Keplerian Disk in the Infalling Envelope around L1527 IRS: Transformation from Infalling Motions to Kepler Motions, ApJ, 796, 131
  72. Ohashi, S., Sanhueza, P., Sakai, N., et al., 2018, Gravitationally Unstable Condensations Revealed by ALMA in the TUKH122 Prestellar Core in the Orion A Cloud, ApJ, 856, 147
  73. Ostriker, J., 1964, The Equilibrium of Polytropic and Isothermal Cylinders., ApJ, 140, 1056
  74. Oya, Y., Sakai, N., Lopez-Sepulcre, A., et al., 2016, Infalling-Rotating Motion and Associated Chemical Change in the Envelope of IRAS 16293-2422 Source A Studied with ALMA, ApJ, 824, 88
  75. Palmeirim, P., Andre, P., Kirk, J., et al., 2013, Herschel View of the Taurus B211/3 Filament and Striations: Evidence of Filamentary Growth?, A&A, 550, A38
  76. Pineda, J. E., Goodman, A. A., Arce, H. G., et al., 2010, Direct Observation of a Sharp Transition to Coherence in Dense Cores, ApJL, 712, L116
  77. Pineda, J. E., Maury, A. J., Fuller, G. A., et al., 2012, The First ALMA View of IRAS 16293-2422. Direct Detection of Infall onto Source B and High-resolution Kinematics of Source A, A&A, 544, L7
  78. Plunkett, A. L., Arce, H. G., Mardones, D., et al., 2015, Episodic Molecular Outflow in the Very Young Protostellar Cluster Serpens South, Nature, 527, 70
  79. Raga, A. C., Canto, J., Calvet, N., Rodriguez, L. F., & Torrelles, J. M. 1993, A Unified Stellar Jet / Molecular Outflow Model, A&A, 276, 539
  80. Safron, E. J., Fischer, W. J., Megeath, S. T., et al., 2015, ApJL, Hops 383: an Outbursting Class 0 Protostar in Orion, 800, L5
  81. Sakai, N., Sakai, T., Hirota, T., et al., 2014, Change in the Chemical Composition of Infalling Gas Forming a Disk around a Protostar, Nature, 507, 78
  82. Sakai, N., Oya, Y., Lopez-Sepulcre, A., et al., 2016, Subarcsecond Analysis of the Infalling-Rotating Envelope around the Class I Protostar IRAS 04365+2535, ApJL, 820, L34
  83. Santiago-Garcia, J., Tafalla, M., Johnstone, D., & Bachiller, R., 2009, Shells, Jets, and internal working surfaces in the molecular outflow from IRAS 04166+2706, A&A, 495, 169
  84. Schnee, S., Enoch, M., Johnstone, D., et al., 2010, An Observed Lack of Substructure in Starless Cores, ApJ, 718, 306
  85. Schneider, S., & Elmegreen, B. G., 1979, A catalog of dark globular filaments, ApJSS, 41, 87
  86. Seo, Y. M., Shirley, Y. L., Goldsmith, P., et al., 2015, An Ammonia Spectral Map of the L1495-B218 Filaments in the Taurus Molecular Cloud. I. Physical Properties of Filaments and Dense Cores, ApJ, 805, 185
  87. Shang, H., Allen, A., Li, Z. -Y., et al., 2006, A Unified Model for Bipolar Outflows from Young Stars, ApJ, 649, 845
  88. Shu, F. H., 1977, Self-similar collapse of isothermal spheres and star formation, ApJ, 214, 488
  89. Shu, F. H., Najita, J. R., Shang, H., & Li, Z. -Y., 2000, X-Winds Theory and Observations, Protostars and Planets IV, 789
  90. Stahler, S. W., 1994, The kinematics of molecular outflows, ApJ, 422, 616
  91. Stahler, S. W. & Palla, F., 2005, The Formation of Stars, Wiley-VCH
  92. Stodolkiewicz, J. S., 1963, On the Gravitational Instability of Some Magneto-Hydrodynamical Systems of Astrophysical Interest. Part III, Acta Astron., 13, 30
  93. Stone, J. M. & Norman, M. L., 1993, Numerical Simulations of Protostellar Jets with Nonequilibrium Cooling. II. Models of Pulsed Jets, ApJ, 413, 210
  94. Tafalla, M., Mardones, D., Myers, P. C., et al., 1998, L1544: A Starless Dense Core with Extended Inward Motions, ApJ, 504, 900
  95. Tafalla, M., Myers, P. C., Caselli, P., Walmsley, C. M., & Comito, C., 2002, Systematic Molecular Differentiation in Starless Cores, ApJ, 569, 815
  96. Tafalla, M., Myers, P. C., Caselli, P., & Walmsley, C. M. 2004, On the internal structure of starless cores. I. Physical conditions and the distribution of CO, CS, $N_2H^+$, and $NH_3$ in L1498 and L1517B, A&A, 416, 191
  97. Tafalla, M. & Hacar, A., 2015, Chains of dense cores in the Taurus L1495/B213 complex, A&A, 574, A104
  98. Tafalla, M., Su, Y.-N., Shang, H., et al., 2017, Anatomy of the internal bow shocks in the IRAS 04166+2706 protostellar jet, A&A, 597, A119
  99. Wilner, D. J., Myers, P. C., Mardones, D., & Tafalla, M., 2000, Small-Scale Structure of the Protostellar Collapse Candidate B335 Imaged in CS J = 5 - 4 Emission, ApJL, 544, L69
  100. Yen, H. -W., Takakuwa, S., Koch, P. M., et al., 2015, No Keplerian Disk >10 AU Around the Protostar B335: Magnetic Braking or Young Age?, ApJ, 812, 129
  101. Zapata, L. A., Loinard, L., Rodriguez, L. F., et al., 2013, ALMA 690 GHz Observations of IRAS 16293-2422B: Infall in a Highly Optically Thick Disk, ApJL, 764, L14
  102. Zhang, Y., Arce, H. G., Mardones, D., et al., 2016, ALMA Cycle 1 Observations of the HH46/47 Molecular Outflow: Structure, Entrainment, and Core Impact, ApJ, 832, 158
  103. Zhou, S., Evans, N. J., II, Koempe, C., & Walmsley, C. M., 1993, Evidence for protostellar collapse in B335, ApJ, 404, 232