Galactic and Extragalactic star formation

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Galactic and Extragalactic star formation

• M.Walmsley (Arcetri Observatory)

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Simplified View of Extragalactic Star Formation

• Based upon
• Assuming local IMF is valid
• everywhere
• Assuming for the SF efficiency some
• version of the Schmidt Law

AMAZING IF TRUE !
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What does galactic star formation tell us?

• Star formation occurs in molecular clouds
• More precisely, it occurs within the dense parts
  of molecular clouds

This raises the question of whether star
formation occurs because the gas is dense or
because it is molecular or both
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Galactic Extragalactic connection

• That there is a connection is clear from the
  various correlations found between star
• formation and molecular line luminosity (most
  recently from Wu et al)
• The HCN luminosity tracks the IR luminosity
  with the same relationship for galactic and
  extragalactic SFR

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HCN luminosity

• In galactic clouds, L(HCN) is thought to
  be
• roughly given by
• L(HCN) ?Cex n(H2) n(HCN) dV
• This involves the HCN abundance and hence the
  Wu et al. result suggests similar chemistry in
  the extragalactic clouds as in galactic.

So the Wu et al. result seems to imply that
extragalactic starbursts are similar to galactic
starbursts but much larger
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Do abundances differ in galactic and
extragalactic clouds?

• Yes ! But there are some rough correspondences as
  one sees comparing NGC253 to galactic clouds
  (S.Martin et al.)

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Simplified view of IMF

• Field Star IMF is within errors same as that
  inferred for ONC (Orion Nebula Cluster) and other
  nearby star forming regions
• It has a power law (Salpeter) down to about 0.5-1
  M(Sun) with most mass in solar mass stars but
  most luminosity at high M
• Evidence for deviations from standard IMF in some
  Gal. Center clusters

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One possible explanation of the IMF

• It reflects the mass distribution of the cloud
  fragments or cores in the molecular cloud
• The typical mass of around 1 MO then reflects
  the Jeans Mass (very T dependent)

M(JEANS) T3/2 n-1/2
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The origin of the Initial Mass Function
Submm continuum surveys of nearby protoclusters
suggest that the mass distribution of
pre-stellar condensations mimics the form of the
stellar IMF
Condensations mass spectrum in r Oph
NGC2068 protocluster at 850 mm
(see also Testi Sargent 1998 Motte et al.
2001)
Motte et al. 2001

• The IMF is at least partly determined by
  fragmentation at the pre-stellar stage.

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Consequences for extragalactic SF

• If fragmentation is fundamental in determining
  the IMF, the Jeans Mass and hence the temperature
  may determine the critical turn-over mass
• This could cause the IMF in galactic nuclei to be
  more biased towards high mass ???
• Temperatures in Galactic Center clouds are high

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The Schmidt Law

• The Schmidt Law for the star formation rate (SFR)
  has many forms

SFR d?/dt ?p with p1-2
Alternatively
d?/dt ?/t(SF)
Where ? is col.density and t(SF) is timescale for
star formation
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Galactic timescale for Star Formation tSF

• One might naturally think it was the free-fall
  time at the mean density of molecular clouds
• But as pointed out in the 70s by Zuckerman and
  Evans, real galactic SF Rate is lower (tSF109
  yr) than from free fall time (tff roughly 106
  years)
• This has given rise to two classes of theories
• slow including ambipolar diffusion modulated
  theories.
• inefficient turbulence, HII regions and winds

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Star Form. Rate in Galactic Dense Clumps

• From Plume et al (1997) the SF rate in galactic
  clouds corresponds to a timescale of 107 to 108
  years - but tff is 105 yr

tff/tSF as in GMCs
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But it cannot be too inefficient

• Some cluster masses are 10 percent of maximum GMC
  masses (Blitz et al., Clark)

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Both slow and ineffcient SF may be needed

• Maybe better to write the Schmidt law

d?/dt ??/ tSF where tSF ß tff
?/? 0.01
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Conclusions

• Extragalactic star formation may well be just
  galactic writ large
• But we do not understand what determines the
  efficiencies and timescales
• Of course the IMF might be playing tricks

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Stars form in spiral arms
M33 Spitzer Image From Verley et al.
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What theory can say about Schmidt Laws

• Mainly that Nature can conspire to make t(SF)
  equal to orbital time scale
• (Tan, McKee and others)
• For example cloud collision rate depends on
  shear which in turn depends on orbital parameters
  ??