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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
al.
20
05
(a
tlas)
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 =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.
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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?