SEMINAR REPORT I: HYPERCOMPACT HII REGIONS

Ref: M. Sewilo, et al, ApJ,605：285-299Tara Murphy, et al, Arxiv:1002.4627v1Stan Kurtz, Proceeding IAU Symposium No. 227

FengLi2010-03-22

PREFACE The formation and earliest evolution of

massive stars is one of astrophysics’ least understood problems.(M. Sewilo, et al., 2004)

CONTENTS Evolution of HII regions HC HII regions

History properties different from UC HII emission properties

Examples Summary

STAR FORMATION EVOLUTION SERIES

Prestellar

coresHot cores

Compact HII

UCHII Classical HII

HYPERCOMPACT HII REGIONS(HC HII)

• 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

COMPARE TO OTHER RELATIVE OBJECTS

Distinguish UC HII and HC HII from EM, density, size, RRL width, rising spectral index, maser association…

ODD(OR INTERESTING) EMISSION PROPERTIES

Rising spectra index of radio continuum spectrum(from short cm to mm)

Broad radio recombination lines(BRRL)

RISING SPECTRA INDEX OF RADIO CONTINUUM SPECTRUM

(Kurtz, 2005)

INDICATION 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

POSSIBLE INTERPRETATION 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

(Sewilo et al, 2004)

ALTERNATIVE THEORIES 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.

ODD(OR INTERESTING) EMISSION PROPERTIES

Rising spectra index of radio continuum spectrum(from short cm to mm)

Broad radio recombination lines(BRRL)

BROAD RADIO RECOMBINATION LINES(BRRLS)

Sewilo et al, 2004

UC HII regions typically show radio recombination line (RRL) widths of , while HC HII have RRL widths

POSSIBLE CAUSES 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.

EXAMPLES OF HC HII

Sewilo et al, 2004

SUMMARY 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?

UC HII

Compact HII HC HII ?

Thank you for attention!