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Hot Cores and High Mass star formation. Malcolm Walmsley (Arcetri Observatory). Orion in NH3 (Mid 70s). Al Barrett and collaborators at MIT noticed a blue shifted component in their NH3 line profiles towards Orion-KL It was most apparent in the highly excited transitions tracing hot gas.

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hot cores and high mass star formation

Hot Cores and High Mass star formation

Malcolm Walmsley

(Arcetri Observatory)

orion in nh3 mid 70s
Orion in NH3 (Mid 70s)
  • Al Barrett and collaborators at MIT noticed a blue shifted component in their NH3 line profiles towards Orion-KL
  • It was most apparent in the highly excited transitions tracing hot gas
nh 3 in 80s
NH3 IN 80s
  • Hot gas is likely compact and so it made sense to observe this “hot core” with the VLA:
  • Genzel et al found the emitting gas was roughly 0.02 pc (10 arc sec) in size
  • More surprisingly the NH3 col. density was enormous, suggesting a large fraction of N in NH3
  • Hermsen et al showed that the NH3 was thermalized at 150 K
why so many saturated species
Why so many saturated species ?
  • As time went on, it became clear that the Orion hot core had high abundances of saturated species (like NH3) but low abundances of radicals.

• The most natural explanation of this together with the high T was in terms of evaporation of ice mantles

• In fact the gas phase chemistry has long time scales to establish equilibrium and so the observed abundances reflect those in the solid state.

• In the solid state, we know from NIR observations that H2O, CO2 .. are abundant

solid state results from nir
Solid state results from NIR
  • If observed Hot Core abundances are due to evaporation, one expects correlation with those observed in ices
  • In fact observations with ISO and from ground show H2O followed by CO2 and CO to be abundant

• But in what form N and S are is still a puzzle

studies of abundances in protostars
Studies of abundances in protostars
  • SWAS, ODIN, ISO have shown that the H2O abundance varies over orders of magnitude from 10-4 in hot (>100K) gas to 10-7 at low T

• Qualitatively similar for methanol (CH3OH) and several other species

• Hence the idea that maps in one of these species map hot gas close to the protostar

what are hot cores useful for
What are hot cores useful for?
  • They trace the hot gas and hence the energy sources
  • But the confusion caused by clustering of protostars is great (outflows add to this)
  • One needs excellent angular resolution to sort this out
infrared observations make a difference
Infrared Observations make a difference
  • For example De Buizer et al. with Gemini

Images of G29.9 show

difficulty of separating

UCHII and hot core or


Red contours HII

Blue 11.7 micron

the ir allows a believable sed fit de buizer et al
The IR allows a believable SED fit (De Buizer et al.)

Fit to 18000 Lsun model

(B1 star, 11 Msun)

Acc rate 0.02 Msun yr-1

what observations of massive protostars say
What observations of massive protostars say
  • Luminosity as function of gas mass depends on fraction of mass in stars
  • Ly Continuum deduced from radio flux often less than expected
what is the central engine doing
What is the central engine doing?
  • What is the nature of the central protostar responsible for heating the hot core gas? Energy from accretion or nuclear burning on ZAMS ?

• From radio observations, we infer that most hot cores have low LyC output

– If nuclear burning, then a cluster of B stars

– If accretion, then one needs an extended,

low Teff atmosphere

orion in different molecular lines
Orion in different molecular lines

Beuther et al. SMA Images, Note coincidence of SMA1 with

vibrationally excited CH3OH, looks like a cluster

ngc 7538
NGC 7538

Kraus et al. NIR


Complex morphology

With many outflows

And cluster of young


the confusion caused by outflows
The confusion caused by outflows

NGC 7538

Kraus et al

A cluster of protostars

produces a cluster of

outflows and one requires

very high angular resolution

to make out individual


what should a massive protostar look like
What should a massive protostar look like
  • Answering this will require theory!
  • Getting round the radiation pressure problem seems to require a massive disk
  • This may imply an outflow as well perhaps as the formation of multiple systems
simulated hot cores
Simulated Hot Cores
  • Krumholz and McKee have simulated an accreting protostar of 35 Msun
  • One sees a map of the gas hotter than 100K
  • Infalling gas is forced by radiation pressure to accrete onto a disk seen edge-on here

Note the “ring” due to material forced to move poloidally

the simpler case of gl2591
The simpler case of GL2591
  • One way forward is to study a “simpler” lower

luminosity hot core such as GL2591 at 1kpc with 104 Lsun. Single protostar ?

Map of 1mm continuum and H218O 203 GHz emission

with IRAM Pl. de Bure

0.8 sec res. (the disk?)

Van der Tak et al. 2005.

the protostellar luminosity increases with age
The protostellar luminosity increases with age
  • Radius of envelope with T > 100 K increases with time (Doty et al. 2006) and likewise radius of increased H2O abundance
  • Applied to GL2591, one derives an age of 5 104years
age determination using hot core lines
Age determination using hot core lines
  • Both the luminosity in hot core lines and size depend sensitively on age and the most luminous are about to become UCHIIs
  • This “age” is NOT the time spent at high T but

rather the time since accretion started

It is likely however to be sensitive to

detailed geometry, outflows etc

future trends
Future Trends
  • Understanding more requires better angular resolution in transitions like the 203 GHz H218O line and the vib.excited methanol
  • Sensitive mid IR imaging (De Buizer) will tell us a lot
  • If we can see in any way (scattered NIR) the central engine(photosphere), that would also be fundamental
conclusions from observed abundances
Conclusions from observed abundances
  • There are families of species which are enhanced together (e.g. CH3CN,C2H5CN, probably HCN,HC3N) perhaps because they froze out together
  • Despite much trying, noone has as yet found a convincing way of using the abundances to determine ages (use as a clock)
  • Most ambitious to date are attempts to use the H2O abundance
ammonia was abundant and thermalised
Ammonia was abundant and thermalised !

Hermsen et al. with the 100-m found T

of order 150 K and high NH3 abundance

the barrier of radiation pressure
The barrier of radiation pressure
  • Radiation pressure was shown by Kahn, Wolfire and Cassinelli and others to be a fundamental barrier to forming a really massive star
  • Solutions proposed have ranged from stellar mergers, changes in dust properties, accretion via a disk