Seminar report I: HYPERCOMPACT HII regions

1. Ref: M. Sewilo, et al, ApJ,605：285-299 Tara Murphy, et al, Arxiv:1002.4627v1 Stan Kurtz, Proceeding IAU Symposium No. 227 Seminar report I: HYPERCOMPACT HII regions FengLi 2010-03-22

2. The formation and earliest evolution of massive stars is one of astrophysics’ least understood problems.(M. Sewilo, et al., 2004) preface

3. Evolution of HII regions • HC HII regions • History • properties different from UC HII • emission properties • Examples • Summary contents

4. Star formation Evolution series Prestellar cores Hot cores UCHII Compact HII Classical HII

5. First discovered by Gaume et al.(1995) • Only a few number of these regions were known until 2005. • the Australia Telescope 20 GHz survey (AT20G), 2004-2008 ;he 2nd epoch Molonglo Galactic Plane Survey • (MGPS-2) at 843 MHz, ~2007 HypercompacthII regions(HC HII)

6. Compare to other relative objects Distinguish UC HII and HC HII from EM, density, size, RRL width, rising spectral index, maser association…

7. Rising spectra index of radio continuum spectrum(from short cm to mm) Broad radio recombination lines(BRRL) Odd(or interesting) emission properties

8. Rising spectra index of radio continuum spectrum (Kurtz, 2005)

9. Density gradient within the gas.Power law density distribution results in power law SED.(Olnon 1975,Panagia& Felli 1975) The turnover frequency is above the highest frequency point, this would imply an emission measure in excess of so that electron density greater than indication

10. Model: an ensemble of clumps of varying optical depths, which can explain the intermediate spectral index over broad frequencyranges(Kurtz, 2005)The alpha range from ,typically (Sewiloet al, 2004) Possible interpretation

11. Photoevaporating accretion disks around massive stars, corresponds well to the sizes of HC HII regions. The disk wind would present a density gradient, which could explain the spectral indices seen. Tan & McKee(2003) propose that the high outflow rates resulting from high accretion rates will produce small, jet-like hypercompact HII regions. Keto(2003) proposes that low accretion rates, with the material passing through a small ionized region where stellar gravity dominates thermal pressure, can produce long-lived, hypercompact HII regions. Alternative theories

12. Rising spectra index of radio continuum spectrum(from short cm to mm) Broad radio recombination lines(BRRL) Odd(or interesting) emission properties

13. Broad radio recombination lines(BRRLs) UC HII regions typically show radio recombination line (RRL) widths of , while HC HII have RRL widths Sewilo et al, 2004

14. Pressure broadening (especially in high density regions) Motions of gas. Including nebular expansion, rotation of a large and massive accretion disk, infall of matter onto an accretion disk, shocks, and bipolar jets of matter from the protostar/disk. Unresolved components, which have BRRL, combined emission will be even broader. Current resolution and sensitivity of the current observations are unable to determine the relative contributions from each mechanism. BTW, the models referred to explain SED, all include broadening mechanism. Possible causes

15. Examples of HC hII Sewilo et al, 2004

18. Discussed HC HII region properties Showed some examples Prospect: Higher resolution observations will resolve the puzzles in HC HIIs: Is HC HII the end of evolution line? Or whether there is something intrinsically different about each class? But a major problem will still puzzle us: What is the source of the trigger for the simultaneous formation of star clusters with large separations in molecular clouds? summary Compact HII UC HII HC HII ?

19. Thank you for attention!