MASSIVE STAR FORMATION

J. Tan (University of Florida, Astronomy, Gainesville, United States), M. Beltran (INAF-Osservatorio Astrofisico di Arcetri, Italy), P. Caselli (University of Leeds, School of Physics & Astronomy, United Kingdom), F. Fontani (INAF-Osservatorio Astrofisico di Arcetri, Italy), A. Fuente (Observatorio Astronomico Nacional, Spain), M. Krumholz (UC Santa Cruz, Astronomy, United States), C. McKee (UC Berkeley, Depts. of Physics & Astronomy, United States), A. Stolte (University of Bonn, Dept. of Astronomy, Germany)

The enormous radiative and mechanical luminosities of massive stars impact a vast range of scales and processes, from the reionization of the universe, to the evolution of galaxies, to the regulation of the interstellar medium, to the formation of star clusters, and even to the formation of planets around stars in such clusters. Furthermore, the synthesis and dispersal of heavy elements by massive stars plays a key role in the chemical evolution of the cosmos. Achieving a rigorous theoretical understanding of massive star formation is thus an important goal of contemporary astrophysics. This effort can also be viewed as a major component of the development of a general theory of star formation that seeks to explain the birth of stars of all masses and from all the variety of star-forming environments. Two main classes of theories for massive star formation are under active study, "Core Accretion" and "Competitive Accretion". In Core Accretion, the initial conditions of star formation are self-gravitating, centrally concentrated cores that condense from the surrounding, fragmenting clump environment with a range of masses. They then undergo relatively ordered collapse via a central disk to form a single star or a small-N multiple. In this case, the pre-stellar core mass function has a similar form to the stellar initial mass function. In Competitive Accretion, the material that forms a massive star is drawn more chaotically from a wider region of the clump without passing through a phase of being in a massive, coherent core. In this case, massive star formation must proceed hand in hand with star cluster formation. If stellar densities become very high near the cluster center, then collisions between stars could also be involved in forming the most massive stars. We review recent theoretical and observational progress towards understanding massive star formation, considering a range of observed galactic star-forming environments, physical and chemical processes, comparisons with low and intermediate-mass stars, and connections to star cluster formation.

Outline

• Observations
• Theory

State of Observations

Plot 1: the Tan plot

• Focusing in <10^4 solar mass region
• IRDCs
• Falgarone size-linewidth relation
• Virial parameter, virial relation.
• Non-equilibrium high-latitude clouds
• GMCs borderline bound

PPV: Clumps form clusters, cores form stars

• Ginsburg 2012: no >10^4 Msun starless clusters
• Butler & Tan 2012: clump vs core surface density

How identify starless cores?

• Chemistry: N2D+ vs N2H+ (Fontani 2011)
• starless cores have higher deuteration (but not always)
• Tan 2013: ALMA observations of starless cores
  • use extinction techniques
  • approximate virial equilibrium in cores
  • assumptions: ions line width, pressure equilibrium, etc.
• H13CO+ cores in Cyg X (Csengeri 2012)
• Many suggestions of virial equilibrium
• Li, Kauffman etc. show many clouds have alpha_vir << 1. Explanations:
  • Maybe strong B-fields?
  • Assumed a fixed CO abundance
  • Short time immediately before free-fall collapse

Theory

• protostars contain many jeans masses
• Turbulence important within core?
• B-fields maybe less important
• Radiative feedback important (Kahn 1974, Starrfield 1971) * force of photons can exceed gravity

Is high-mass accretion a scaled-up version of low-mass star formation?

Yes: (Turbulent Core)

• mass of star related to mass of core
• But: Why not fragment further?
• Where are protostellar accretion disks?

No: (Competitive Accretion)

• tidally modified bondi-hoyle accretion
• Doesn't work in virialized turbulent medium
• B-fields reduce accretion

Radiation pressure still a problem?

Direct collisions?

• While forming, or after forming
• Sana 2012: expect high-mass binaries to merge
• Moeckel & Bonnell: merger theory
• but they don't affect observed clusters (Moeckel 2011)

Radiation suppresses fragmentation

• column threshold for massive star formation
• B-fields plus radiation effective at suppressing fragmentation (Commercon 2011)

G35.2N Gemini / SOFIA observations: Zhang 2013

Rowan Smith 2013: predicted ALMA observations

Peters 2010: fragmentation induced starvation Keto 2007: bondi-hoyle accretion

Radiation escape: beaming, cavities

Observational questions:

• Does the IMF mirror the core mass function?
• Are disks present in high-massprotostars?
• Protostellar luminosity function for massive stars?
• Is IMF at high end universal?
• How do you make very massive >150 msun stars?
• Can massive stars form in isolation?

Why is there a difference?

• initial conditions
• missing physics

Questions

• Q: Doug (Johnstone?) - Core mass function: thermal -> 1 star, turbulent -> ??? stars. Observationally, is the column the important quantity?
• A: Yes, theoretically. Cores won't necessarily make single star (or system). Massive cores are likely to form high and low-mass stars. Under what conditions will most of the mass go in to one or two stars?
• Q: Radiation feedback during formation. How does this gel with massive stars forming in close binaries?
• A: All simulations have terrible resolution. Radiative + magnetic suppresses fragmentation
• Q Ewine: Radiation feedback... 1 g/cm^2. What scale?
• A: Up to 1000 AU, on larger scale B-field must be more effective.
• Q Klessen?: [mic problems] Caution about using virial theorem. Complex density-velocity structure. Surface terms should not be neglected.
• A: If you look at any object and compare to virial, you need time-dependent and surface terms. Still a good way to assess gravity.
• Surface term can explain virial parameters...
• No it can't...
• Q: Hiro? (Taiwan) - Observations. Not signatures of direct accretion onto stars?
• A: Most accretion was on to star-disk system. How do you get it onto the star? Yes, that's one of the open difficult questions. Observations within 10 AU of star.
• Q: Rolf Kuiper - Radiative tail instability. Our simulations were confirmed last year by two groups.... [not a question]
• A: I agree that flux limited diffusion underestimates force. Zheng also disagrees with you
• Q: Mike Kuhn (sp?): How does the presence of active formation of low-mass stars prior to massive stars affects the core processes?
• A: Universality of IMF only true averaged over spacetime. Low-mass stars drive turbulence in massive star-forming region.