VLASS – Galactic Science
Life cycle of star formation in our Galaxy
as a proxy for understanding the Local
Universe
legacy science
Infrared GLIMPSE survey
VLASS – Galactic Science
Detailed study and Identification of High
Mass Star Formation Regions
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high mass stars have a huge effect on their
environs, through UV radiation, wind energy,
shocks, enriching the ISM
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our understanding of high mass star formation
is woefully incomplete: triggered star
formation vs. spontaneous collapse?
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these stars drive winds and jets, which are
compact thermal sources
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H II regions form once the star has contracted
down to the main sequence, and are useful
probes of high mass star formation
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methanol masers are associated with regions
of high mass star formation
HST image of NGC 3603, showing a cluster
of massive star formation
GLIMPSE and CORNISH (5 GHz survey)
VLASS – Galactic Science
Stellar Winds, Planetary Nebulae, and Active Stars
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thermal radio emission from stellar winds complements UV, optical studies
evolved OB stars reside in the densest part of the Galaxy
understanding mass loss  mass and energy flows in local Universe
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PN have flat radio spectra in the GHz range (methanol masers reveal physical conditions)
only 3000 (of proposed 23,000) planetary nebula currently known (Frew & Parker 2010)
rates of detection and mass loss rates affect models for input energy into ISM
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Magnetically active stars have flat radio spectra from non-thermal emission with possible
gyroresonance at higher frequencies; high spatial resolution enables identification
Detection of a previously unknown
planetary nebula from the CORNISH
survey, Hoare et al. (2012). Left: 5 GHz
Middle: Spitzer GLIMPSE (3.6,4.5,8 um)
Right: UKIDSS (JHK)
VLASS – Galactic Science
Setting the Landscape
Ku band survey is complementary with many other
survey efforts
VLASS – Galactic Science
example: constraining thermal sources
With the proposed sensitivity,
radio emission can be used to
distinguish between these types
of thermal sources in star forming
regions Hoare et al. (2012)
 radio loud UCHII regions
 radio quiet Massive YSOs
 CORNISH limited in scope!
 sensitivity and spatial coverage
Typical spectral energy distribution of an Ultra-Compact HII
region (red curve). The distinctive shape is the product
of two emission processes: thermal radiation from warm
dust (the 'hump') and thermal free-free radio emission
from ionised gas (the 'plateau'). Fluxes normalized
VLASS – Galactic Science
• Key Science Advantages of Ku band
– sensitivity is optimized to thermal sources (a > +0.6)
– access to 12.2 GHz methanol maser (usually associated with 6.7 GHz
methanol maser and star formation) to help constrain pumping
mechanism; access to recombination lines
– complementary with infrared and submillimeter surveys that trace dusty
regions of early and current star formation
– picks up thermal emission from dense plasmas which are missed by lower
frequencies
• Key Science Disadvantages of S band
– baseline sensitivity of 30 mJy will not pick up the same population as the
baseline sensitivity at Ku band will: miss many optically thick, thermal
sources!
– non-thermal (AGN) contamination will make source identification nature
very difficult at S-band
– no spectral lines (methanol maser very important indicator of star forming
regions; recombination lines provide basic kinematics)
VLASS – Galactic Science
Parameters of Proposed Survey
Galactic Latitude Coverage