1. Stellar content of visibly obscured 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) W31 G23.96+0.1 5

2. Outline 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

3. Direct stellar signatures If A V ~few, O star spectral types (T eff ) are obtained from blue visual spectra e.g. HeI 4471/HeII 4542 (Walborn 1971) If A V ~20-30 mag, near-IR spectral lines may be used instead, e.g. HeII 1.692  m/HeI 1.700  m (Hanson et al. 1998; Lenorzer et al. 2004) Conti & Alschuler 1971 Fit to dwarfs (  ) from Hanson et al. (2005) Conti & Frost 1977

4. Indirect stellar signatures For high A V, 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 (T eff ) 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] 12.8  m/[NeIII] 15.5  m) together with photo- ionization models (CLOUDY) should allow estimate of T eff for the ionizing star(s).

5. U T eff Problems? Predicted nebular fine- structure line ratios depend sensitively upon T eff and…. Martin-Hernandez et al. 2002 30kK 35kK 40kK Simon-Diaz & Stasinska 2008 Ne + S 2+ metallicity; stellar atmosphere models. n e or U (= N LyC /(4  R S 2 n e c) ); Metal rich Metal poor

6. Metallicity dependence Martin-Hernandez et al. 2002 Metal-poor; high ionization Metal-rich; low ionization GC Orion 30 Dor

7. G29.96-0.02 (UCHII) T eff =32-35kK (late O) from CMFGEN + nebular analysis of ([NeIII]/[NeII]; Martin- Hernandez et al. 2002; Morisset et al. 2002) T eff =41  2 kK (O4-5V) from an analysis of near-IR spectrum (Hanson et al. 2005 IAUS 227), feasible since A K ~2 mag Need more cases, but typically compact clusters lie within HII regions. Ionizing stars of UCHII regions rarely seen in near-IR.

8. 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). Hanson et al. 2005 (atlas) 2MASS JHK 10” (0.25 pc @ 5kpc) ISAAC 2.2  m

9. Stellar Cluster W31 (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)

10. Near- & mid-IR spectroscopy Refined spectral types for 5 W31 cluster members from VLT/ISAAC O3-5.5V for 4 “naked” O stars (~30-55 M o ) 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.

11. Mid-IR diagnostics U dependence separated from T eff using U T eff Significant differences between empirical mid-IR line ratios & metal- rich CMFGEN + CLOUDY models predictions If n e known,

12. Calibration of UCHII 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.

13. 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 Gemini Michelle IR S 2E W51d1 OKYM2 IRS2W [SIV]/[NeII]~0.5

14. Extragalactic HII regions Relevant to interpretation of mid-IR data for starburst regions e.g. IC4662 (Gilbert & Vacca 2008)

15. Starbursts [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.

16. Summary In principle, ratios of mid-IR fine structure lines offer means of establishing Sp Types (T eff ) 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.

17. Mid-IR diagnostics Simon-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 N LyC /(4  R S 2 n e c). 35 40 45 -3 -2 U=N LyC /(4  R 0 2 n e c).

18. 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 models?