Transcript Hot Cores and High Mass star formation

Galactic and Extragalactic star
formation
M.Walmsley (Arcetri Observatory)
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 !
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
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
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
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.)
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
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
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
NGC2068 protocluster at 850 mm
Motte et al. 2001
Condensations mass spectrum in r Oph
(see also Testi & Sargent 1998; Motte et al. 2001)
 The IMF is at least partly determined by fragmentation at the pre-stellar
stage.
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
The Schmidt Law
• The Schmidt Law for the star formation rate (SFR)
has many forms:
SFR = d∑/dt ~∑p with p=1-2
Alternatively :
d∑/dt ~∑/t(SF)
Where ∑ is col.density and t(SF) is timescale
for star formation
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 (tSF=109 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
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
But it cannot be too inefficient
• Some cluster masses are 10 percent of maximum
GMC masses (Blitz et al., Clark)
Both “slow” and “ineffcient” SF
may be needed
Maybe better to write the Schmidt law:
d∑/dt = ∑/ tSF
where tSF = ß tff
/ ~ 0.01
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
Stars form in spiral arms
M33 Spitzer Image
From Verley et al.
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 ??