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School of somethingFACULTY OF OTHER
School of Physics & AstronomyFACULTY OF MATHEMATICS & PHYSICAL SCIENCES
Massive YSOs and the transition to UCHIIsMelvin Hoare
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Outline
• Definition of MYSOs
• Ionized jets and winds
• Definition of UCHIIs
• Why MYSOs do not ionize their surroundings
• RMS Survey population synthesis
• Diagnostic Plots
• Morphologies
• Conclusions
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Massive Young Stellar Objects
• Luminous (>104 L) embedded IR point source
• bipolar molecular outflow (~10 km s-1)
• ionised wind (~100 km s-1)
• no UCHII region
GL 2591
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• MYSOs display weak radio emission
• A few have been resolved to show jets
• Proper motions show velocities ~500 km s-1
Ionized Jets
Cep A2 (Patel et al. 2005)
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• Others show evidence of radiation driven disc wind
Disc winds
S140 IRS 1 (Hoare 2006)Drew, Proga & Stone (1998)
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Wind Spectra
Gibb & Hoare (2007)
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IR line wind diagnostics
• IR H I recombination lines are formed in the same gas that emits the radio continuum (e.g. Hoeflich & Wehrse 1987)
• Ratios of Brackett series lines indicate multiple components: fast optically thick outflow and a narrower optically thinner component
S106IR (Lumsden et al. in prep)
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Spectro-astrometric jet detection
W33A Davies et al. (2010)
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IR line disc diagnostics
• Fe II line and CO bandhead formed in dense, neutral material close to star – most likely a disc
Blum et al. (2004)
Lumsden et al. (in prep)
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Two views of a disc
• CO bandhead also arises in disc
• Broader in direct view (edge-on) than in reflected (face-on) view
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Definition of UCHIIs
• In a UCHII the central star is ionizing the surrounding interstellar material and not material driven from the star/disc system
MIR dust emission (de Buizer et al. 2002)G29.96-0.02from Megeath et al.
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Cometary HII Regions
• Exponential density gradient, O9V stellar wind and proper motion of 10 kms-1 up density gradient (Arthur & Hoare 2006)
Emission measure at i=45o Velocity structure of nebula & wind
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Why do MYSOs not ionize their surroundings?
• Walmsley (1995) suggested that infall quenches the HII region – effectively making it very high EM and therefore not seen in radio
• However, likely to still be seen in near-IR recombination lines since
• But we do not see very strong, relatively narrow NIR lines
• Should also see many bipolar UCHIIs if star has ionizing flux would still escape down the outflow cavity, but we do not.
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MYSO stars are not hot!
• MYSOs do not ionize their surroundings to form a UCHII region as they are swollen by ongoing accretion and therefore have Teff<30 000 K
• No MYSOs above L=105 L (M~30 M) as they rapidly contract to MS radii and therefore have Teff>30 000 K
• Test with population synthesis of the RMS survey of MYSOs and UCHIIs
Hosokawa & Omukai (2008)
Hosokawa & Omukai (2009)
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RMS Population Synthesis
• Distribute in the spiral arm model (Cordes & Lazio) n
• Sample from a Kroupa IMF
• Assume an accretion rate history
• Transition to UCHII when on ZAMS and Strömgren expansion thereafter
• Include selection criteria F21>MSX completeness limit (~3 Jy), <20
• Compare to total Galactic star formation rate (~3 Myr -1)
Davies et al. in prep
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Accretion Rate History
McKee & Tan (2003) Schmeja & Klessen (2004)
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Evolutionary Tracks
Hosokawa priv comm.
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Increasing Accretion Rate
tMM fin
McKee & Tan (2003)
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Decreasing Accretion Rate
tteM
log
Schmeja & Klessen (2004)
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Evolution
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Transition Objects
• Still predicts that stars above ~ 30 solar masses are accreting whilst in the UCHII phase
• Some HII region exciting stars exhibit MYSO spectral features of accretion like the CO bandhead
• A few very young bipolar HII regions found such as NGC 7538 IRS 1
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Diagnostic plots: Size vs linewidth
• High frequency lines narrower
• No distinction between UCHIIs and HCHIIs
• HCHII x UCHII o MYSO
Hoare et al. (2007) PPV
G28.20-0.04N (Keto et al. 2008)
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Radio vs IR luminosity
• Clear distinction between UCHIIs and MYSOs at luminous end
• MYSOs also distinguished from OB star winds – MS OB stars not detected yet
Jets Evolved OB stars Hoare & Franco (2007)
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Radio to IR ratio vs speed
• Big distinction between UCHIIs and MYSOs
• HWZI is a lower limit to wind speed
Hoare & Franco (2007)
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‘HCHII’ Morphologies - Cometary
G24.78+0.08 A1 (Beltran et al. 2007)G34.26+0.15 B (Avalos et al. 2008)
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‘HCHII’ Morphologies - Shells
G28.20-0.04N (Sewilo et al. 2008) +RRLsG34.26+0.15 B (Avalos et al. 2008)
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Bipolar – Transition Object?
• NGC 7538 IRS 1 is bipolar and variable
(Franco-Hernandez & Rodriguez (2004)
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Outflow not infall
• Velocity structure indicates bipolar flow is expanding and not contracting as well as having a decreasing radio flux
(Kraus et al. (2006)
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Conclusions
• The vast majority of HCHIIs are just smaller, younger versions of UCHIIs
• Not a distinct class of object with different physical process at work
• Not to be confused with MYSO winds and jets
• However, hyper-compact bipolar HIIs may be important transition objects
• e-Merlin, EVLA, MeerKAT high resolution studies may find more of these, but they will be very rare
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Mm Dust Emission
Integrated
Peak 24”
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Modelling H II Region Dust Emission
G45.13+0.14A Hoare et al. (1991)
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Multiple Sources in Beam