Physics 778 (2009): Star formation

1. Overview

Ralph PudritzOrigins Institute and Dept. of Physics & AstronomyExt. 23180pudritz@physics.mcmaster.caOffice; ABB 31

Turbulent ISM: Canadian Galactic Plane Survey (CGPS): the turbulent interstellar medium: shocks driven by supernova explosions and stellar winds

(Atomic H map: Midplane of Milky Way - near Perseus)

Star formation in the Galaxy

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.

Radio

Optical

Infrared

X-ray

Same Space -- Different Light

The Galactic Center in visible Light

Major questions in star formation:

Macroscopic aspects: 1. How is the filamentary and clumpy structure of self-

gravitating clouds related to star formation?2. What determines the stellar mass spectrum (the

“initial mass spectrum” IMF)? Is it universal?3. How do star clusters form?

Micropscopic aspects:4. How do individual stars/disks/jets form?5. Do low and high mass stars form in the same way?6. How did the first stars in the cosmos form?

1. Observational Overview - Macroscopic Aspects: Stars form in massive clouds of dusty, cold, molecular gas

- To detect gas - map millimetre wave emission from carbon monoxide molecule.

- To detect dust - map sub-millimetre emission from dust grains (eg. Use James Clerk Maxwell Telescope – on top of Mauna Kea volcano - Hawaii)

Optical images and infrared images of the Orion Nebula

IRAS satellite: sensitive at wavelengths 10 – 100 microns

Structure of Giant Molecular Clouds (GMCs) on 100 pc scale

Extinction map of Orion and Mon clouds (Cambresy 1998) –rightScuba continuum 850 micron map of 10 pc portion of cloud (Johnstone

& Bally 2006)

(Carpenter et al 2009)

Taurus molecular cloud – site of low mass star formation (distance: 140 pc. )

FCRAO – molecular survey in 12 and 13 CO (top, bottom). Integrated intensity maps. 45” resolution, 1 km/sec velocity resolution

Highly filamented structure, with cavities and rings also apparent

Narayan et al 2008, ApJS

Taurus properties Alignment of magnetic

field (measured by optical polarimetry) with molecular streaks

Cloud mass 2x10^4 solar masses

Low efficiency of star formation

SFE = mass of young stars/ molecular mass =0.3 – 1.2 %

(General feature of molecular clouds is that they are in bulk very inefficient in forming stars.) Goldsmith et al 2008

Extragalactic studies of clouds: M51 – Whirlpool Galaxy

Global spiral waves and associated star formation.

Molecular clouds associated with dust seen in HST image - right

GMCs in M33 – Association with HI filaments

Engargiola et al, 2003, ApJS

- Catalogue of 148 GMCs, complete down to 1.5 x 10^5 solar masses.

-Steep mass spectrum for clouds with index

(-2.6)

Filaments and star clusters: clusters of stars form in special places: hub -filament systems (Myers 2009)

Above: Rho-Oph

Right: Pipe Nebula

The Origin of Stellar Masses: Formation of Molecular Cloud Cores?

(Motte et al 2001)

• Numerous small dense gas “cores” within a clump. Individual stars form in cores – typically 0.04 pc in size

Filaments – home to cores, stars, clusters…

Megeath et al – Spitzer data

c2d Spitzer legacy results:

90% of stars lie within loose clusters.

(Evans et al 2009, ApJS

Orion GMC - and the Orion Nebula Cluster

Most stars form as members of star clusters and not in isolation:

Major clue to origin of the Mass Spectrum of stars (or initial mass function, IMF)

Scale ONC ~ 1 pc

N II B in the Large Magellanic Cloud, Hubble Space Telescope Heritage

Super-massive star clusters

Core Mass Function (CMF). Note it has similar form as the Initial Mass Function (IMF) for stars.

Stellar mass spectrum - the “initial mass function” (IMF)- Broken power laws(Salpeter)

at high mass: - 1.35 if plotted with log M*)

- Lognormal + power law (Cabrier 2003, Hennebelle + Chabrier 2007)

Link between CMF and IMF:

Kroupa 2002, ScienceAlves et al 2007 (Pipe Nebula)

2. Microphysics of star formation: Gravitational collapse and formation of a star/disk/jet system

Infrared image Barnard 68 (Alves et al 2001): excellent fit with Bonner-Ebert model (pressure truncated isothermal sphere)

Disks around young – and old stars

Submm image of Epsilon Eridanni Greaves et al (1998)

Orion Proplyd – star in formation

T-Tauri Stars – Spectral Energy Distributions Young T-Tauri stars

have Spectral Energy Distributions (SEDs) that differ from pure photospheric models of young star – pronounced Infrared Excess observed.

This is attributed to emission from an accretion disk.

Feature evolves with time as disk disappears ( over a few million years) D’Alessio et al 1999

Measure thrust

in swept-up CO;

(Cabrit & Bertout1992)

Correlation works for both low and high mass stars

For 391 outflows: Wu et al (2004) same index

3.03 )10/(250/

LL

cL

Fbol

bol

CO

Molecular (CO) outflows

High speed optical jets - are strongly correlated with disk properties

Evidence for jet/disk coupling: (i) jet rotation

(Bacciotti et al 2003, Coffey et al 2004, Pesenti et al 2004)

jet rotation, 110 AU from source, at 6-15 km/sec

Footpoints for launch of jet *extended over disk surface* (Anderson et al 2003) LV originates from

disk region: 0.3-4.0 AU

(ii) accretion and jet mass loss rates coupled (wide

variety of systems (eg. Hartmann et al 1998)

1.0/

aw MM

Stellar spins: rotation properties of stars in Orion Nebula Cluster (Herbst et al 2002)

Spins of 396 stars in cluster measured… (spotted TTSs)

- Slow rotators correlate with IR excess – indicates presence of disk.

-bimodal… massive stars spin slowly at 8 days, and have a high spin peak at 2 days.

- Low mass stars have a single peak, 2 days

Unified models for stellar spin (Matt & Pudritz, ApJL, 2005)

Question: why do young stars rotate

so slowly?

- MHD wind maintains stellar

spin at small values through accretion

powered wind

2*

2* )/( RrRM Aww

Gas Accretion & Gap-formation

HH 30 (from HST)

Flared, gaseous, dusty disk

http://www.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html

Protoplanet

Star formation and planet formation closely linked

Forming the first star….

First stars, formed in primordial gas, devoid of metals, dust, strong B field,…

Going back in time – to 400 million years after the Big Bang – to the “dark era”

In what kind of objects did such stars form, and how did they influence their environment…

Text

Cosmic reionization End of the cosmic dark ages

measured by Ly alpha absorption of background quasars (Fan et al 2006) – “Gunn-Peterson” effect.

First (massive) stars as ionizers?

Spectra of quasar “probes” – redshifts 5.74<z<6.42

Left: Lyman alpha optical depth as function of redshift

Formation of the first star

Physics is much simpler – no magnetic fields, no dust or complex molecules, primordial gas contains hydrogen and helium.

Start with a small, cold, dark matter halo – about a million times mass of the Sun.

Gas within it cools down to 200 K, (molecular hydrogen is the coolant)

A 100 solar mass core forms inside a filamentary “molecular cloud”

First stars turned on perhaps 400 million years after the Big Bang… they started to ionize and enrich the IGM. First steps towards planets and ultimately life...

How do we observe all of this? Canada’s new international facilities..

New Observatories:- James Webb Space Telescope - Atacama Large Millimetre Array (50 telescope millimetre array)

ALMA and JCMT will resolve disks, find forming Jupiters and first stars.

(SEE: www.casca.ca/lrp)

Birth of a Solar System: what ALMA can do…..

ALMA band 7 300 GHz = 1 mm

resolution = 1.4” to 0.015”

100 AU = 0.3” at d=300pc

~ Highest resolution at 300 GHz = 1 mm (0.015”)

~ Highest resolution at 850 GHz = 350 m