Download Document

Chemistry and dynamics of the pulsating starless core
Barnard 68
Matt Redman
National University of Ireland, Galway
Matt Redman
NUI Galway
Collaborators
• Eric Keto (CfA, Harvard, USA)
• Jonathan Rawlings (University College London, UK)¸
• David Williams (University College London, UK)
Matt Redman
NUI Galway
Star formation models
The many models can be divided into two types
1. Dynamic/turbulent (e.g. Klessen et al 2000; Padoan &
Norlund 2002; Ballesteros-Palledes 2003)
•
Rapid formation of interacting protostellar cores from
density fluctuations generated by supersonic turbulence.
Stars ‘freeze out’ as energy dissipated
2. Hydrostatic Shu model (e.g. Shu 1977; Shu, Adams &
Lizano 1987)
•
Initially stable cloud gradually loses pressure support
(thermal and/or magnetic) against gravity and collapses
Matt Redman
NUI Galway
Observing molecular cloud cores
• Stars form inside cold (~10 K) dense (~106 cm-3) dusty ‘cores’
in molecular clouds
• These cores are highly obscured so need to observe in mm or
sub-mm regimes
• Continuum observations detect the dust emission
• Line observations (from gas phase molecules or ions) trace the
gas and its dynamics
Matt Redman
NUI Galway
Barnard 68
• A Bok globule discovered in 1919 by Barnard
• Contains a few solar masses of molecular gas and is
gravitationally bound
• Can we determine whether Barnard 68 (and objects like it) will,
or will not, form a star?
• Where does it fit into the turbulent or hydrostatic scenarios
Matt Redman
NUI Galway
Barnard 68
Initial density structure
Very direct approach carried out by Alves et al (2001)
•
Measure reddening of background stars through core to
sample extinction at thousands of positions across the face of
the core
•
Relies on the core being projected against a suitable star field
such as the galactic bulge
•
Can then make an extinction map
Matt Redman
NUI Galway
Initial density structure
•
Obtain a radial extinction profile by azimuthally averaging
the sampled positions
•
Model the profile to find central density
Matt Redman
NUI Galway
Initial density structure
•
Density profile well matched by a Bonner-Ebert sphere
(Bonner 1956, Ebert 1955)
•
A Bonner-Ebert sphere is a self-gravitating cloud in
hydrostatic equilibrium with a surrounding pressurised
medium
•
Density profile can be roughly approximated as
Matt Redman
NUI Galway
Bonner-Ebert spheres
•
Described by a modified Lane-Emden equation:
where
Matt Redman
NUI Galway
Bonner-Ebert spheres
•
Solution defined by a single parameter which contains the
radius, central density and sound speed
If
collapse
then the sphere is unstable to gravitational
• B68 appears to be just about stable with a central density of
~few x 105 cm-3
Matt Redman
NUI Galway
Initial density structure
Couple of objections to the Bonner-Ebert modelling:
•
Almost any shaped cloud can then be fitted by spherical
geometries - should fit with a 3D code (Steinacker et al)
•
Transient structures in turbulent star formation models may
also be fitted in this way even though not hydrostatic
(Ballesteros-Paredes et al 2003)
•
Temperature not constant and other sources of pressure
available (magnetic)
Matt Redman
NUI Galway
Temperature structure
•
Dust heated by starlight and then re-radiates at longer
wavelengths
•
Can map the dust emission in the mm/sub-mm continuum
(e.g. using SCUBA Shirley et al 2001; Evans et al 2001)
•
Make assumptions about the dust properties (such as the
opacity) to then model the dust density structure using dust
radiative transfer codes
•
Gives a fit to the temperature profile: cores are warmer at the
edge than the centre. Bianchi et al (2003) measure ~14 K
outer regions, dropping to ~9 K inner regions
Matt Redman
NUI Galway
Stability of cores
•
Recently suggested by Casselli et al (2004, in press) that
cores should be divided as follows according to whether
temperature and density are greater or less than Tc~10 K and
nc ~ 105 cm-3
Stable starless cores - those whose Bonner-Ebert parameter
suggests they will never become stars (‘failed cores’). These
cores will not dissipate as expected in the turbulent models.
Unstable pre-stellar cores - those whose parameters indicate they
are unstable against collapse and may well show evidence of
the onset of collapse
Matt Redman
NUI Galway
Is Barnard 68 stable?
•
Barnard 68 is not cold enough and dense enough to suggest
imminent collapse
•
Is it a transient or long lived structure?
•
If it is stable, there should be no collapse signature
Matt Redman
NUI Galway
Chemical age
•
The centre of Barnard 68 is heavily depleted in CO and CS
•
N2H+ survives in the central regions
•
The depletion implies that B68 is a long-lived structure:
freeze-out occurs in the centre first and moves outwards
•
Supported by dust emissivity measurements by Bianchi et al
(2003) that suggest dust is composed of ice covered
coagulated grains
•
B68 must have been dense and cold long enough for freezeout to occur
Matt Redman
NUI Galway
Dynamics
•
Rich chemistry in molecular clouds initiated by cosmic rays.
Lines excited by CMB.
•
Use gas phase rotational lines to probe the kinematics of the
interiors of the cloud
•
Shape and strength of the line profile can reveal the internal
velocity structure
Matt Redman
NUI Galway
Line profiles and dynamics
•
Evans (1999) reviews the line profile shape expected from a
collapse model (such as the Shu model)
•
Expect a double-peaked line profile with the blue wing
stronger
•
Simple molecules such as CS and HCO+ are observed - not
too abundant so not too optically thick and chemically ‘well
behaved’
•
If a blue asymmetric profile is seen then the rare isotope (e.g.
H13CO+) is checked - should be optically thin single peaked
gaussian
Matt Redman
NUI Galway
Kinematics of B68
•
If B68 is a stable starless core, should be no evidence of
collapse or outflow
•
But it shows strange alternating pattern of red and blue
asymmetric profiles
Matt Redman
NUI Galway
Red
Red
Blue
Blue
Red
CS (2-1) Lada et al (2001)
Red
HCO+ 3-2
Origin of velocity pattern
•
Alternating pattern of red and blue asymmetric profiles
confirmed in another molecule
•
Turbulence of outer layers of cloud one possibility
•
Or some combination of rotation, infall and freeze-out
•
Keto & Field (2005) describe an instability in dusty BonnerEbert spheres - may be applicable to B68
Matt Redman
NUI Galway
Instability in Bonner-Ebert spheres
•
Consider a cloud in which the density is low and close to that
of dust-gas thermal coupling
•
Cosmic rays heat the dust but the low density means the dust
does not efficiently warm the gas - temperatures of dust and
gas de-couple
•
If the cloud is perturbed by an increase in external pressure,
the cloud contracts but the warmer dust heats the gas. The
cloud then reexpands
•
Can set up an oscillation in the outer layers
Matt Redman
NUI Galway
Application to B68
•
This effect could be occuring in a non-uniform way in B68:
some parts of the cloud are expanding, others contracting
•
If correct then stable starless cores could display ‘infall’
signatures that are in fact caused by instabilities in the cloud
•
As long as the oscillations are not too severe, the cloud will not
collapse and will be long-lived
•
True infall indicated by infall signatures in the most volatile
species such as N2H+ that trace the central gas plus a central
density and temperature on the unstable side of the BonnerEbert solution
Matt Redman
NUI Galway
Conclusions
•
The temperature and density of Barnard 68 indicate that it is
gravitationally stable - it is not dense enough and too warm to
collapse
•
The chemistry indicates that it is not a transient structure sufficient time with comparable physical conditions must have
elapsed to allow freeze-out to take place
•
However the core is not quiescent but dynamically active in a
complex but not entirely random way
•
Barnard 68 is unlikely to form a star unless external conditions
change - it is a stable starless core
Matt Redman
NUI Galway