Stellar Content of HII Regions

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					    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)

G23.96+0.                                  W31
15

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               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
      Direct stellar signatures
If AV~few, O star
spectral types (Teff) are
obtained from blue                                      QuickTime™ and a
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visual spectra e.g. HeI                                                                    QuickTime™ and a
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4471/HeII 4542
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(Walborn 1971)
       Fit to dwarfs ()                             Conti & Alschuler 1971
       from Hanson et al.                             Conti & Frost 1977
                 (2005)
                                           If AV~20-30 mag,
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                                           near-IR spectral lines
                                           may be used instead,
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                                           e.g. HeII 1.692 m/HeI
                                           1.700m (Hanson et al.
                                           1998; Lenorzer et al.
 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).
                                              Problems?
  Predicted nebular fine-
  structure line ratios depend                                                                 Metal
  sensitively upon Teff and….                                                                  poor
• ne or U (= NLyC/(4RS2nec));                                                                            QuickTime™ an d a
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• metallicity;
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                                                                                                                                        Metal
• stellar atmosphere models.                                                                                                            rich
                                                                                                                                      Ne+
                                                                                                                              S2+
       Simon-Diaz &                                                                        Martin-Hernandez et al. 2002
       Stasinska 2008
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               U
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                  Teff
                                                      Q uic kT ime ™ a n d a
                                                        d ec om p re s so r
                                                                        s
                                              a re n ee d ed to s ee th i p ictu re .




   30kK                                                                             35kK                    40kK
     Metallicity dependence
                  Martin-Hernandez et al.
                   2002          30 Dor

                                                                 Metal-poor;
                                                                 high ionization

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                                                         Orion
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                 GC

Metal-rich;
low ionization
       G29.96-0.02 (UCHII)
Teff=32-35kK (late O)
from CMFGEN +
nebular analysis of                     QuickTime™ an d a
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([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 more cases, but
                        Need mag
                        typically compact clusters lie
                        within HII regions. Ionizing
                        stars of UCHII regions rarely
       G23.96+0.15 (UCHII)
    2MASS                                                          ISAAC
    JHK                                                            2.2m




                                                                                                                 Hanson et al. 2005
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     2’=3pc@5kp
     c                                                  10” (0.25 pc @ 5kpc)

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.
   Stellar Cluster W31 (GHII)
K-band
spectroscopy from
Blum, Damineli &
Conti (2001)
revealed a young
stellar cluster within                QuickTime™ and a
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W31 (G10.2-0.3) at             are neede d to see this picture.



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
 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
                                             .eru tcip sih t ee s o t d edeen e ra
                                                      ros serpmoced
                                                  a dna ™emi Tk ciu Q




  ~1.5 Myr, plus O6V for            QuickTime™ and a
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  a massive YSO (source
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  26).
• Spitzer/IRS reveals
  highest [NeIII]/[NeII]
  ratios for “naked” stars
  Greatly mass, quickest
• (highestexpanded sample
  to shed dust nebular plus
  with mid-IR cocoon?)
  near-IR stellar datasets.
                Mid-IR diagnostics
U dependence
separated from                                                 U
Teff using
      [SIV]/[SIII]                                             Teff

     [NeIII]/[NeII]
 If ne known,
     1
                      1/
       ne B N LyC  3          QuickTime™ and a

 U                
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     c  36 
Significant
differences
between empirical
mid-IR line ratios
& metal-rich
CMFGEN +
CLOUDY models
  Calibration of UCHII regions?
Ground-based
   mid-IR
   spectroscopy
   limited to
   [SIV]/[NeII].
In this case,               QuickTime™ and a

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   offset between
   observation and
   prediction.
For metal-rich HII
  regions
  calibration may
  be possible.
          G49.49-0.37 (W51A)
• N-band imaging of
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  ~30 UCHII regions
                                      [SIV]/[NeII]~
                                          Gemini
  often reveals multiple              0.5
  (dust) continuum                        Michelle
  sources                OKYM
                         2          W51d1
• Spectral types of
       [SIV]/[NeII]~
  individual stars may                        IRS2W QuickTime™ and a
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       0.1
  be extracted from             IRS
  [SIV]/[NeII] ratios
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                                2E
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• First attempted in this
  context by Okamoto
  et al. (2003) for       8 arcsec = 0.2 pc (@
  G70.29+1.60             5.5kpc)
      Extragalactic HII regions
                                                                         QuickTime™ an d a
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   Relevant to
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interpretation
     of mid-IR
       data for                                    QuickTime™ and a
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  regions e.g.
        IC4662
    (Gilbert &
 Vacca 2008)
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                                         Starbursts
                                               [NeIII]/[NeII] ratio is
                                               used to deduce stellar
                                               content/IMF/age of
                                               starbursts (e.g.
            QuickTime™ and a
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                                               Thornley et al. 2000).
      are neede d to see this picture.




                                                           QuickTime™ an d a
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Essential to ensure
photoionization models
are well calibrated.
               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
         Mid-IR diagnostics
Simon-Diaz & Stasinska                                         Unfortunately agreement
(2008) appeared to                                             is lost for solar grid, once
(nearly) resolve                                               U has its usual definition
stellar/nebular                                                NLyC/(4RS2nec).
discrepancy for G29.96-
             35
0.02
                                                40

                                                     45
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                                                          -1
                                                -2

                                           -3

U=NLyC/(4R02nec).
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)

				
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posted:3/3/2013
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