The Early Phase of Star Formation
The Big Questions
Questions extracted from SF research with the aim to identify a set of "big questions"
Stars matter for astronomy.
We work hard to learn
 
how they are formed.
Source
The questions are gathered from published reviews and papers, presentations, and discussions. Names and citations are intentionally not given to keep personal biases out.
Definition: what means "big"?
Without answering a "big" question it is not possible to explain the SF process. In the SF field, the entangled action of many physical processes makes the application of this definition especially difficult. A substantial fraction of the listed questions may actually turn out to be not a "big" question.
Highly disputed
Since research funding is favorably provided for break-through achievements in the big questions of a field, their precise content is a matter of intense scientific and science-political discussion.
For a few questions, a likely answer accepted by a large fraction of the community exists (green text).
No technical questions
There are important questions addressing how to perform the research in order to be able to answer the scientific questions. These can range from observational questions about the telescope, observing techniques, and data reduction over fitting algorithms or pattern recognition to questions related to simulations and theoretical models. This list only governs the scientific questions.
Input to this list
If you have a comment about this list, a paper or pdf with potentially big questions not mentioned here, a request to change wording, or a wish to add a question or an answer, you can send it as email to       .
Last change: 10 June 2024 09:47 CET (97 questions)
Abbreviations:   SF=Star Formation,   (G)MC=(Giant) Molecular Cloud,   IRDC=InfraRed Dark Cloud,   SN=SuperNova

G e n e r a l
  
Ge01
Are low-mass stars formed by a different physical process than high-mass stars?
Ge02
Is there a sequence in the SF process in which low-mass SF, high-mass SF, and feedback act in a certain order?
Ge03
What is the origin of the initial mass functions measured in different locations of the Galaxy?
Ge04
What regulates the SF efficiency/rate?
Ge05
Fe11
Which effects hinder or terminate SF leading to the observed low Galactic SF efficiencies?
Ge06
Is there a global column density limit for SF to start?
Ge07
Fi05
Do filaments have a universal thickness, potentially related to the Jeans mass?
Ge08
In which phase and by which physical processes is the multiplicity of newly-formed stars determined?
Ge09
MC15
How do the properties of MC and the SF in their interior depend on galactic environment?
Ge10
What is the role of metallicity both in the SF process and in conversions from observations to properties?
Ge11
Is there a limit for the final mass of the formed star and how does it depend on the metallicity in the forming cloud?
Ge12
Fe12
How does the evolving protostar affect the matter accreting on it?
Ge13
Fe13
Can radiation stop the accretion flow?
Ge14
Fe14
What is the fraction of stars being formed by triggered SF?
Ge15
What is the fraction of high-mass stars that evolve through stellar merging?
Ge16
MC17
What mechanism solves the angular momentum problem at MC scales?
Ge17
What is the main driver of the turbulence that is affecting SF regions?
Ge18
At what scale does turbulence influence gas and dust and what is its effect?
Ge19
From the molecular cloud down to the inner border of an accretion disk, where are the regions that are magnetically dominated?
Ge20
Which effect on SF can be expected from large-scale MHD waves?
Ge21
Does ambipolar diffusion have a measurable effect on SF?
Ge22
In which phase is the SF process considerably affected by cosmic rays?
Ge23
What are the cosmic dust properties in the SF regions and how does our limited knowledge about them hinder us to derive physical quantities?
Ge24
Does the formation and destruction of molecular ices play any role in the early SF process?

M o l e c u l a r   C l o u d s
  
MC01
Can MCs be formed in minima of the galactic gravitational potential?
MC02
Can MCs be formed by galactic shearing motions or colliding flows?
MC03
Can MCs be formed by expanding clustered SN shells?
MC04
In view of galaxies without spiral arms showing GMCs/SF, are spiral arms necessary to form MCs?
MC05
Fi01
Which of the observed filamentary structure of MCs is created by gravity, which by shearing motions, which by colliding flows, and which by clustered SN shells?
MC06
Fe08
Fi02
Are O and B stars able to create filamentary structures of MCs by their winds?
MC07
Are the magnetic fields strong enough to impact the MC formation and evolution?
MC08
Fe15
Can magnetic fields channel flows to filaments or affect their orientation?
MC09
What is the fraction of MCs that undergo gravitational collapse?
MC10
Are MCs in static virial equilibrium?
MC11
How much gas can a dense region in a GMC accumulate before SF starts?
MC12
Do all GMCs form stars?
MC13
What are the timescales for the formation of the MC, its filamentary structure, the cores, and the MC destruction, and how are they related?
MC14
How much do the conditions to form MCs change near the Galactic center compared to the disk, and how much is that reflected in their properties?
MC15
Ge09
How do the properties of MC and the SF in their interior depend on galactic environment?
MC16
Fe07
Do MCs interact with other MCs?
MC17
Ge16
What mechanism solves the angular momentum problem at MC scales?

F i l a m e n t s
  
Fi01
MC05
Which of the observed filamentary structure of MCs is created by gravity, which by shearing motions, which by colliding flows, and which by clustered SN shells?
Fi02
Fe08
MC06
Are O and B stars able to create filamentary structures of MCs by their winds?
Fi03
What fragments filaments?
Fi04
What determines the lifetime of a filament?
Fi05
Ge07
Do filaments have a universal thickness, potentially related to the Jeans mass?
Fi06
How do filaments accrete from their surrounding and which forces aside of gravitation affect this accretion?
Fi07
Is the mass flow through converging filaments the dominant process to form clusters?
Fi08
What causes the striation in filaments?
Fi09
How essential for the filament mass and evolution is the mass supply by striation?
Fi10
What causes the fibers in filaments?
Fi11
Do all filaments have fibers?
Fi12
How are magnetic fields affecting the filament evolution?
Fi13
Co01
What causes the appearance of cores in filaments?
Fi14
Co17
Is there a relation between the filament properties and the core properties?
Fi15
Co02
Are there cores that have not formed in a filament?
Fi16
Is the formation of dense clumps at the end of filaments (EDC, end-dominated filaments) a common process?
Fi17
Co18
Can the properties of future cores already be deduced from the filament properties?
Fi18
Co16
What are the precise physical definitions of filaments/clumps/fragments/cores?

C o r e s
  
Co01
Fi13
What causes the appearance of cores in filaments?
Co02
Fi15
Are there cores that have not formed in a filament?
Co03
Which processes stabilize the core?
Co04
What causes the core collapse?
Co05
Is the core collapse in quasi-equilibrium or a pressureless free-fall?
Co06
Why does turbulence weaken towards the center of the cores?
Co07
Do all cores (even in IRDCs) start as a low-mass core with about a thermal Jeans mass and then grow by competitive accretion?
Co08
Is it possible to destroy cores (e.g. by large-scale shearing motions) before they can create a star?
Co09
How important is core collision in the overall math of SF?
Co10
Is there a relation between the core properties and the properties of the created stellar population?
Co11
Are there cores with a single density peak that have enough mass to produce an O star?
No. When increasing observational spatial resolution, all massive clumps show multiple peaks.
Co12
How does the combined action of competitive accretion and fragmentation lead to the observed variety in core mass functions?
Co13
Do cores already show mass segregation within young stellar clusters?
Co14
Fe06
Cores near a forming cluster: will the radiation and winds trigger SF or prevent it?
Co15
How strong is the effect of non-ideal MHD effects on the evolution of the core?
Co16
Fi18
What are the precise physical differences between filaments/clumps/fragments/cores?
Co17
Fi14
Is there a relation between the filament properties and the core properties?
CO18
Fi17
Can the properties of future cores already be deduced from the filament properties?

D i s k s
  
Di01
Do all stars have a phase where they accrete mass through a circumstellar disk?
Di02
Which physical mechanisms provide viscosity in the disk aiding to accrete matter onto the star?
Di03
Je03
How much of the angular momentum of the disks is removed by the jet and the outflow?
Di04
Which process accretes more mass onto the star: accretion events or low-rate constant accretion?
Di05
Are there extended (thousands of AU) massive (disk/star mass ratio not ≪1) disks around high-mass proto-stars?
Only a few cases are known. Disks with a size and mass scaled-up from low-mass disks are expected to be unstable due to self-gravity, asymmetric accretion flows, the action of magnetic fields, and the gravitational interaction with nearby high-mass density peaks/protostars.
Di06
How much mass is accreted through disk-instabilities?
Di07
How common are accretion streamers?
Di08
How much is the disk disturbed by the infall through streamers?
Di09
Is protostellar disk formation and evolution regulated magnetically?
Di10
Fe16
Which processes dissolve the accretion disk?
Di11
Fe17
What is the effect of stellar winds on the disk formation and evolution?
Di12
What is the effect of multiplicity on the formation and evolution of disks?
Di13
What is the connection between the disk thickness and the accretion process?
Di14
Is there a relation between the disk orientations and the global magnetic field directions in an MC with SF?
Di15
Does planet formation take place in protostellar disks already?

J e t s / O u t f l o w s / W i n d s
  
Je01
Which mechanisms launch the jet and the outflow from the star-disk system?
Je02
What mechanism collimates the jet?
Je03
Di03
How much of the angular momentum of the disk/star system is removed by the jet and the outflow?
Je04
How much turbulence is injected by the jets and the outflows into the SF region?
Je05
What is causing explosive outflows?
Je06
Are the jet knots created by pulsed injection or interaction with the ambient medium?
Je07
Fe18
What is the impact of the jets and outflows on dense gas near the SF site?
Je08
How much do the jet and disk wind properties vary with the stellar mass?
Je09
Why are jets observed only in a small fraction of the lifetime of the disk?

F e e d b a c k
  
Fe01
Which effect has the first SN in a proto-cluster on the SF?
Fe02
How was the SF region formed that created our sun?
Fe03
What is the effect of the shock wave of a SN cluster on the local part of a galaxy and the SF within?
Fe04
How do the cosmic rays that are created in the SF process affect the SF region?
Fe05
Can (M)HD waves produced during the SF process affect the SF in the region?
Fe06
Co14
Cores near a forming cluster: will the radiation and corresponding heat transfer as well as winds trigger or prevent SF?
Fe07
MC16
Do MCs interact with other MCs?
Fe08
MC06
Fi02
Are O and B stars able to create filamentary structures of MCs by their winds?
Fe09
Which effect has the ionization of gas near a protostar or proto-cluster on the accretion and the nearby SF?
Fe10
Can the numerous B stars energetically compete with their wind feedback against the strong feedback of a few SNs?
Fe11
Ge05
Which effects hinder or terminate SF leading to the observed low Galactic SF efficiencies?
Fe12
Ge12
How does the evolving protostar affect the matter accreting on it?
Fe13
Ge13
Can radiation stop the accretion flow?
Fe14
Ge14
What is the fraction of stars being formed by triggered SF?
Fe15
MC08
Can magnetic fields channel flows to filaments or affect their orientation?
Fe16
Di10
Which processes dissolve the accretion disk?
Fe17
Di11
What is the effect of stellar winds on the disk formation and evolution?
Fe18
Je07
What is the impact of the jets and outflows on dense gas near the SF site?
The end is near!