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)

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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)

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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 photo- ionization models (CLOUDY) should allow estimate of Teff for the ionizing star(s).

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Teff

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

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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

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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

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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).

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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 a

l. 20

05

(atla

s)

2MASS JHK

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10” (0.25 pc @ 5kpc)

ISAAC ISAAC 2.22.2mm

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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.

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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?)

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

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

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η =[SIV ]/[SIII]

[NeIII]/[NeII]

U

Teff

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

€

U = 1cneα BNLyC

36π ⎛ ⎝ ⎜

⎞ ⎠ ⎟

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.

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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

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8 arcsec = 0.2 pc (@ 5.5kpc)

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[SIV]/[NeII]~0.1

GeminiMichelle

IRS 2E

W51d1

OKYM2

IRS2W

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[SIV]/[NeII]~0.5

Extragalactic HII regionsExtragalactic HII regions

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Relevant to interpretatio

n of mid-IR data for

starburst regions e.g.

IC4662 (Gilbert &

Vacca 2008)

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

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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

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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?