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Massive YSOs and the transition to UCHIIs

School of Physics & Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES. Massive YSOs and the transition to UCHIIs. Melvin Hoare. Outline. Definition of MYSOs Ionized jets and winds Definition of UCHIIs Why MYSOs do not ionize their surroundings RMS Survey population synthesis

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Massive YSOs and the transition to UCHIIs

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  1. School of Physics & Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES Massive YSOs and the transition to UCHIIs Melvin Hoare

  2. 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

  3. 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

  4. Ionized Jets • MYSOs display weak radio emission • A few have been resolved to show jets • Proper motions show velocities ~500 km s-1 Cep A2 (Patel et al. 2005)

  5. Disc winds • Others show evidence of radiation driven disc wind S140 IRS 1 (Hoare 2006) Drew, Proga & Stone (1998)

  6. Wind Spectra Gibb & Hoare (2007)

  7. 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)

  8. Spectro-astrometric jet detection W33A Davies et al. (2010)

  9. IR line disc diagnostics • Fe II line and CO bandhead formed in dense, neutral material close to star – most likely a disc Lumsden et al. (in prep) Blum et al. (2004)

  10. Two views of a disc • CO bandhead also arises in disc • Broader in direct view (edge-on) than in reflected (face-on) view

  11. 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.

  12. 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

  13. 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.

  14. Hosokawa & Omukai (2009) 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)

  15. RMS Population Synthesis • Distribute in the spiral arm model (Cordes & Lazio) a 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), f<20² • Compare to total Galactic star formation rate (~3 M¤yr -1) Davies et al. in prep

  16. Accretion Rate History McKee & Tan (2003) Schmeja & Klessen (2004)

  17. Evolutionary Tracks Hosokawa priv comm.

  18. Increasing Accretion Rate McKee & Tan (2003)

  19. Decreasing Accretion Rate Schmeja & Klessen (2004)

  20. Evolution

  21. 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

  22. Diagnostic plots: Size vs linewidth • High frequency lines narrower • No distinction between UCHIIs and HCHIIs G28.20-0.04N (Keto et al. 2008) • HCHII x UCHII o MYSO Hoare et al. (2007) PPV

  23. 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 Hoare & Franco (2007) ¨ Jets p Evolved OB stars

  24. Radio to IR ratio vs speed • Big distinction between UCHIIs and MYSOs • HWZI is a lower limit to wind speed Hoare & Franco (2007)

  25. ‘HCHII’ Morphologies - Cometary G24.78+0.08 A1 (Beltran et al. 2007) G34.26+0.15 B (Avalos et al. 2008)

  26. ‘HCHII’ Morphologies - Shells G28.20-0.04N (Sewilo et al. 2008) +RRLs G34.26+0.15 B (Avalos et al. 2008)

  27. Bipolar – Transition Object? • NGC 7538 IRS 1 is bipolar and variable (Franco-Hernandez & Rodriguez (2004)

  28. 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)

  29. 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

  30. Mm Dust Emission Integrated Peak 24”

  31. Modelling H II Region Dust Emission G45.13+0.14A Hoare et al. (1991)

  32. Multiple Sources in Beam

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