Compact HII regions toward Methanol Maser traced sources of Massive Star

Formation

Adam Avison (UK ARC, JBCA)Gary Fuller + MMB Collaboration

Overview

• HII Regions in sites of MSF• The MMB Survey• MMB Continuum sources characterisitcs• Size differences as evolutionary indication• Conclusion

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HII Regions in sites of MSF• Only massive stars are capable of

creating embedded HII region making them unique tracers of massive star formation.

• Ultra Compact HII regions are created as a forming massive star begins ionising the hydrogen in its surrounding natal cloud.

• Hyper Compact HII regions form prior to UCHII and are studied as a separate class of object potentially created as either outflow cavities or the accretion disks are photoionized.

Source Type Diameter[parsec]

EM[pc cm-6]

HCHII ≤ 0.05 109

UCHII 0.05 < l ≤ 0.1 107

HII > 0.1 107

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Values from Kurtz 2002.

The Methanol MultiBeam Survey• The MMB survey was an Anglo-

Australian collaboration which surveyed the galactic plane for class II methanol masers at 6.67GHz.

• The survey region was:l = -174° < l < 60° |b| < 2° and was conducted using the Parkes radio telescope.

• The class II CH3OH maser is uniquely associated with sites of massive star formation (Minier et al 2003).

• ~350 new maser sources were detected during the survey.

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High Resolution MMB Observations

• Using ATCA all newly detected masers were observed to give high resolution positions.

• Correlator setup to simultaneously observe at ~6GHz and 8GHz

• The 8GHz observations were used to look for HII regions in the same field as CH3OH maser sources.

• Of the 414 maser sources in the MMB-ATCA data 105 sources were found at 8GHz.

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Associated Objects?The obvious question to initially ask about these sources is are the Masers and HII regions associated?

Naively: “Some but not all”

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1’’ @ 5kpc = 0.02pc 1000” @ 5kpc = 24pc

Maser/HII separation

[arcsec]

Number of Sources

≤ 1” 11

≤ 10” 21

10”< x ≤ 10’ 73

Characterizing Sources • The sources were fit as 2D

Gaussians with MIRIAD task IMFIT and the integrated flux densities calculated.

• 83 HII regions were well fitted, with the remainder were show extended morphologies.

• From these fits or averaged radial sizes we calculated source diameters.

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Characterizing Sources : The Devil is in the Distance

• Sources with <10” separations from masers distance used are from (Green & McClure-Griffiths 2011 in prep).

• Remaining sources used the near/far kinematic distances (Green 2008).

• Distance ambiguity!

• We used a couple of techniques to try and overcome this.

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Characterizing Sources:Basic Emission Measures

A simplistic emission measure was calculated for each source:

Which was then used to further characterize sources.

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MMB HII region types

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Source Type NumberHCHII 12

UCHII 14

HII 48

Confused/TBC 31

Ever so slightly subject to change. See A.Avison et al. 2011 in prep.

Evolutionary TraitsFrom the Evolutionary time lines of e.g. Breen et al. 2010 one would expect HII regions associated with masers to be smaller in size than those with greater maser/continuum separation.

This effect has been seen by e.g. Walsh et al. 1998 & Ellingsen et al. 2005.

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Breen et al. 2010

K-S Testing for size differencesDistance dependent

• Comparison of diameters for objects with separations of ≤2” and >2” at both distances.

• p-values: 0.5314(near) / 0.6022(far)

Distance independent• Compared integrated-to-peak

flux ratio for same split:

• p-value: 0.4158

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Random Distances

• Using a galactic HII region distribution derived from 354 sources (Caswell & Haynes 1987; Kolpak et al. 2003; Fish et al. 2003; Sewilo et al. 2004; Thompson et al. 2006) each HII region was randomly assigned a distance and the distance dependent test repeated.

• 10,000 iterations were conducted and only 1.5% of them show any difference between the ≤2” and >2” groups.

Conclusions• 105 8GHz continuum sources in the MMB-ATCA data• We categorize them as 12 HCHII, 14 UCHII and 48 HII with 31

still confused.• No size differences seen between associated and large

separation sources. Not entirely surprising.

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Things I haven’t said…

• GLIMPSE characteristics of these sources have been derived.• Interesting velocity information has been seen in the MMB

masers themselves.