Stellar content of visibly obscured HII Regions

1. Stellar content of visibly obscured HII Regions W31 G23.96+0.15 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)

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. Conti & Frost 1977 If AV~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) Direct stellar signatures If AV~few, O star spectral types (Teff) are obtained from blue visual spectra e.g. HeI 4471/HeII 4542 (Walborn 1971) Fit to dwarfs () from Hanson et al. (2005) Conti & Alschuler 1971

4. Indirect stellar signatures • 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] 12.8m/[NeIII] 15.5m) together with photo- ionization models (CLOUDY) should allow estimate of Teff for the ionizing star(s).

5. U Metal poor Simon-Diaz & Stasinska 2008 Teff Metal rich 30kK 35kK 40kK Ne+ S2+ Martin-Hernandez et al. 2002 Problems? • ne or U (= NLyC/(4RS2nec)); Predicted nebular fine-structure line ratios depend sensitively upon Teff and…. • metallicity; • stellar atmosphere models.

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

7. Teff=41  2 kK (O4-5V) from an analysis of near-IR spectrum (Hanson et al. 2005 IAUS 227), feasible since AK~2 mag 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) Need more cases, but typically compact clusters lie within HII regions. Ionizing stars of UCHII regions rarely seen in near-IR.

8. ISAAC 2.2m Hanson et al. 2005 (atlas) 2’=3pc@5kpc 10” (0.25 pc @ 5kpc) G23.96+0.15 (UCHII) 2MASS JHK One exception is G23.96+0.15 (UCHII). VLT ISAAC spectroscopy reveals T~38  1 kK (O7.5V) confirming subtype from low res data (Hanson et al. 2002).

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 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.

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

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. [SIV]/[NeII]~0.5 OKYM2 W51d1 [SIV]/[NeII]~0.1 IRS2W IRS 2E 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 Gemini Michelle 8 arcsec = 0.2 pc (@ 5.5kpc)

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 (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.

17. Mid-IR diagnostics Unfortunately agreement is lost for solar grid, once U has its usual definition NLyC/(4RS2nec). Simon-Diaz & Stasinska (2008) appeared to (nearly) resolve stellar/nebular discrepancy for G29.96-0.02 35 40 45 -1 -2 -3 U=NLyC/(4R02nec).

18. Stellar atmosphere models? 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)