Massive YSOs and the transition to UCHIIs

School of somethingFACULTY OF OTHER

School of Physics & AstronomyFACULTY OF MATHEMATICS & PHYSICAL SCIENCES

Massive YSOs and the transition to UCHIIsMelvin 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 (>104 L) embedded IR point source

• bipolar molecular outflow (~10 km s-1)

• ionised wind (~100 km s-1)

• no UCHII region

GL 2591

• 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)

• Others show evidence of radiation driven disc wind

Disc winds

S140 IRS 1 (Hoare 2006)Drew, Proga & Stone (1998)

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

Blum et al. (2004)

Lumsden et al. (in prep)

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

MIR dust emission (de Buizer et al. 2002)G29.96-0.02from Megeath et al.

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

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 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)

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

Accretion Rate History

McKee & Tan (2003) Schmeja & Klessen (2004)

Evolutionary Tracks

Hosokawa priv comm.

Increasing Accretion Rate

tMM fin

McKee & Tan (2003)

Decreasing Accretion Rate

tteM

log

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

• 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)

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)

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

G24.78+0.08 A1 (Beltran et al. 2007)G34.26+0.15 B (Avalos et al. 2008)

‘HCHII’ Morphologies - Shells

G28.20-0.04N (Sewilo et al. 2008) +RRLsG34.26+0.15 B (Avalos et al. 2008)

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