Hot Cores and High Mass star formation

1. Hot Cores and High Mass star formation Malcolm Walmsley (Arcetri Observatory)

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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 HMPO Red contours HII Blue 11.7 micron

9. Molecular lines in G29 (Cesaroni et al)

10. 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

11. 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

12. 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

13. Orion in different molecular lines Beuther et al. SMA Images, Note coincidence of SMA1 with vibrationally excited CH3OH, looks like a cluster

14. NGC 7538 Kraus et al. NIR Images Complex morphology With many outflows And cluster of young protostars

15. 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 features

16. 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

17. 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

18. 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.

19. 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

20. 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

21. 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

22. 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

23. Ammonia was abundant and thermalised ! Hermsen et al. with the 100-m found T of order 150 K and high NH3 abundance

24. 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