stellar content of visibly obscured hii regions paul crowther (sheffield) james furness (sheffield),...

Stellar content of visibly Stellar content of visibly obscured HII Regionsobscured HII Regions

Paul Crowther (Sheffield)James Furness (Sheffield), Pat Morris (CalTech), Peter Conti (JILA), Bob Blum (NOAO), Augusto Damineli (IAG-USP), Cassio Barbosa (UNIVAP),

Schuyler van Dyk (CalTech)W3W311

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G23.96+G23.96+0.150.15

OutlineOutline

• Direct & indirect stellar signatures in obscured compact HII regions

• Role of mid-IR fine structure lines• G23.96+0.15 (UCHII) & W31 (giant

HII)• Calibration of UCHII regions?• Relevance to starbursts

Direct stellar Direct stellar signaturessignaturesIf AV~few, O star

spectral types (Teff) are obtained from blue visual spectra e.g. HeI 4471/HeII

4542 (Walborn 1971)





If AV~20-30 mag, near-IR spectral lines may be used instead, e.g. HeII 1.692 m/HeI 1.700m (Hanson et al. 1998; Lenorzer et al. 2004)

Conti & Alschuler 1971Fit to dwarfs () from Hanson et al.

(2005)



Conti & Frost 1977

Indirect stellar Indirect stellar signaturessignatures• For high AV, need to rely upon indirect

methods using the ionized gas, e.g. thermal bremsstrahlung emission • Radio continuum flux provides estimate of N(LyC),

yet without any information on the hardness (Teff) of the EUV radiation field.

• Reliable, unless dust absorbs a significant fraction of Lyman continuum photons, and/or free-free emission is not optically thin at observed .

• Mid-IR fine structure lines (e.g. [NeII] 12.8m/[NeIII] 15.5m) together with photoionization models (CLOUDY) should allow estimate of Teff for the ionizing star(s).


are needed to see this picture.U

Teff

Problems?Problems?Predicted nebular fine-structure line ratios depend sensitively upon Teff and….



Martin-Hernandez et al. 2002



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30kK 35kK 40kK

Simon-Diaz & Stasinska 2008

Ne+

S2+

• metallicity;

• stellar atmosphere models.

• ne or U (= NLyC/(4RS2nec));

Metal rich

Metal poor



Metallicity dependenceMetallicity dependenceMartin-Hernandez et al. 2002

Metal-poor; high ionization

Metal-rich; low ionization

GC

Orion

30 Dor

G29.96-0.02 (UCHII)G29.96-0.02 (UCHII)Teff=32-35kK (late O) from CMFGEN + nebular analysis of ([NeIII]/[NeII]; Martin- Hernandez et al. 2002; Morisset et al. 2002) Teff=41 2 kK (O4-5V) from an

analysis of near-IR spectrum (Hanson et al. 2005 IAUS 227), feasible since AK~2 mag



Need more cases, but typically compact clusters lie within HII regions. Ionizing stars of UCHII regions rarely seen in near-IR.

G23.96+0.15 (UCHII)G23.96+0.15 (UCHII)

One exception is G23.96+0.15 (UCHII).



2’=3pc@5kpc

VLT ISAAC spectroscopy reveals T~38 1 kK (O7.5V) confirming subtype from low res data (Hanson et al. 2002).

Han

son

et

al.

20

05

(a

tlas)

2MASS JHK



10” (0.25 pc @ 5kpc)

ISAAC ISAAC 2.22.2mm



Stellar Cluster W31 Stellar Cluster W31 (GHII)(GHII)

K-band spectroscopy from Blum, Damineli & Conti (2001) revealed a young stellar cluster within W31 (G10.2-0.3) at d~3kpc, comprising “naked” O stars & massive YSO’s Ghosh et al. (1989) also identify a number of UCHII regions.



1 arcmin (1 pc @ 3.3 kpc)

Near- & mid-IR Near- & mid-IR spectroscopyspectroscopy• Refined spectral types

for 5 W31 cluster members from VLT/ISAAC

• O3-5.5V for 4 “naked” O stars (~30-55 Mo) with ~1.5 Myr, plus O6V for a massive YSO (source 26).

• Spitzer/IRS reveals highest [NeIII]/[NeII] ratios for “naked” stars (highest mass, quickest to shed dust cocoon?)



• Greatly expanded sample with mid-IR nebular plus near-IR stellar datasets.

Mid-IR diagnosticsMid-IR diagnosticsU dependence separated from Teff using



€

η =[SIV ]/[SIII]

[NeIII]/[NeII]

U

Teff

Significant differences between empirical mid-IR line ratios & metal-rich CMFGEN + CLOUDY models predictions

€

U =1

c

neα BNLyC36π

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 3

If ne known,

Calibration of UCHII Calibration of UCHII regions?regions?

Ground-based mid-IR spectroscopy limited to [SIV]/[NeII].

In this case, systematic offset between observation and prediction.



For metal-rich HII regions calibration may be possible.

G49.49-0.37 (W51A)G49.49-0.37 (W51A)• N-band imaging of ~30

UCHII regions often reveals multiple (dust) continuum sources

• Spectral types of individual stars may be extracted from [SIV]/[NeII] ratios

• First attempted in this context by Okamoto et al. (2003) for G70.29+1.60



8 arcsec = 0.2 pc (@ 5.5kpc)



[SIV]/[NeII]~0.1

GeminiMichelle

IRS 2E

W51d1

OKYM2

IRS2W



[SIV]/[NeII]~0.5

Extragalactic HII regionsExtragalactic HII regions



Relevant to interpretatio

n of mid-IR data for

starburst regions e.g.

IC4662 (Gilbert &

Vacca 2008)





StarburstsStarbursts[NeIII]/[NeII] ratio is used to deduce stellar content/IMF/age of starbursts (e.g. Thornley et al. 2000).





Essential to ensure photoionization models are well calibrated.

SummarySummary• In principle, ratios of mid-IR fine structure

lines offer means of establishing Sp Types (Teff) of ionizing stars in obscured HII regions;

• We provide an increased sample of HII regions, associated with individual O stars, for which both mid-IR nebular diagnostics & spectral types are known (G23.96+0.15, W31);

• In practice, disappointing agreement between observed [NeII-III], [SIII-IV] ratios & expectations from photo-ionization models;

• Nevertheless, [SIV]/[NeII] ratio does have the potential to serve as a diagnostic for HII regions within the inner Milky Way.

Mid-IR diagnosticsMid-IR diagnosticsSimon-Diaz & Stasinska (2008) appeared to (nearly) resolve stellar/nebular discrepancy for G29.96-0.02





Unfortunately agreement is lost for solar grid, once U has its usual definition NLyC/(4RS

2nec).

35

40

45

-3

-2-1

U=NLyC/(4R02nec).

From comparison with ISO observations of HII regions, Morisset et al (2004) concluded:

-CoStar too hard at high energies (approximate treatment of blanketing)

-TLUSTY & Kurucz too soft at high energies (due to neglect of stellar winds)

-CMFGEN & WM-basic in “reasonable agreement” with observations (although they fared no better than a blackbody! SED)

Stellar atmosphere Stellar atmosphere models?models?

stellar content of visibly obscured hii regions paul crowther (sheffield) james furness (sheffield),...

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