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 (~10 6 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: Matt Redman NUI Galway where Bonner-Ebert spheres Solution defined by a single parameter which contains the radius, central density and sound speed Matt Redman NUI Galway If then the sphere is unstable to gravitational collapse B68 appears to be just about stable with a central density of ~few x 10 5 cm -3 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 T c ~10 K and n c ~ 10 5 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 freeze- out 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. H 13 CO + ) 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 CS (2-1) Lada et al (2001) Red Red Blue Blue Red 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 Bonner- Ebert 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 N 2 H + that trace the central gas plus a central density and temperature on the unstable side of the Bonner- Ebert 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