School of Physics
something
& Astronomy
FACULTY OF MATHEMATICS
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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 (>104 L)
embedded IR point source
• bipolar 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=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) 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<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
G28.20-0.04N (Keto et al. 2008)
• High frequency lines
narrower
• HCHII
x UCHII
Hoare et al. (2007) PPV
o MYSO
• 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