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 • Diagnostic Plots • Morphologies • Conclusions

Massive Young Stellar Objects • Luminous (>10 4 L  ) embedded IR point source • b ipolar molecular outflow (~10 km s -1 ) • ionised wind (~100 km s -1 ) • no UCHII region GL 2591

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)

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

Wind Spectra Gibb & Hoare (2007)

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)

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

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)

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

Definition of UCHIIs • In a UCHII the central star is ionizing the surrounding interstellar material and not material driven from the star/disc system G29.96-0.02 from Megeath et al.

MIR dust emission (de Buizer et al. 2002)

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

=45 o Velocity structure of nebula & wind

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.

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 T eff <30 000 K • No MYSOs above L=10 5 L  (M~30 M  ) as they rapidly contract to MS radii and therefore have T eff >30 000 K • Test with population synthesis of the RMS survey of MYSOs and UCHIIs Hosokawa & Omukai (2008) Hosokawa & Omukai (2009)

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 F 21 >MSX completeness limit (~3 Jy), f <20  • Compare to total Galactic star formation rate (~3 M  yr -1 ) Davies et al. in prep

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

Evolutionary Tracks Hosokawa priv comm.

Increasing Accretion Rate 

M



M fin t

McKee & Tan (2003)

Decreasing Accretion Rate log 

M



te



t

Schmeja & Klessen (2004)

Evolution

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

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

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 p Evolved OB stars Hoare & Franco (2007)

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

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

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

Bipolar – Transition Object?

• NGC 7538 IRS 1 is bipolar and variable (Franco-Hernandez & Rodriguez (2004)

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)

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

Mm Dust Emission Integrated Peak 24”

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

Multiple Sources in Beam