The Shapes of Molecular Cloud Cores in Orion

We investigate the intrinsic shapes of starless cores in the Orion GMC, using the prestellar core sample of Nutter and Ward-Thompson (2007), which is based on submillimeter SCUBA data. We employ a maximum-likelihood method to reconstruct the intrinsic distribution of ellipsoid axial ratios from the axial ratios of projected plane-of-the-sky core ellipses. We find that, independently of the details of the assumed functional form of the distribution, there is a strong preference for oblate cores of finite thickness. Cores with varying finite degrees of triaxiality are a better fit than purely axisymmetric cores although cores close to axisymmetry are not excluded by the data. The incidence of prolate starless cores in Orion is found to be very infrequent. We also test the consistency of the observed data with a uniform distribution of intrinsic shapes, which is similar to those found in gravoturbulent fragmentation simulations. This distribution is excluded at the 0.1% level. These findings have important implications for theories of core formation within molecular clouds.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter

A new method for probing magnetic field strengths from striations in the interstellar medium

Recent studies of the diffuse parts of molecular clouds have revealed the presence of parallel, ordered low-density filaments termed striations. Flows along magnetic field lines, Kelvin-Helmholtz instabilities and hydromagnetic waves are amongst the various formation mechanisms proposed. Through a synergy of observational, numerical and theoretical analysis, previous studies singled out the hydromagnetic waves model as the only one that can account for the observed properties of striations. Based on the predictions of that model, we develop here a method for measuring the temporal evolution of striations through a combination of molecular and dust continuum observations. Our method allows us to not only probe temporal variations in molecular clouds but also estimate the strength of both the ordered and fluctuating components of the magnetic field projected on the plane-of-the-sky. We benchmark our new method against chemical and radiative transfer effects through two-dimensional magnetohydrodynamic simulations coupled with non-equilibrium chemical modelling and non-local thermodynamic equilibrium line radiative transfer. We find good agreement between theoretical predictions, simulations and observations of striations in the Taurus molecular cloud. We find a value of $\rm{27 \pm 7} ~\rm{\mu G}$ for the plane-of-sky magnetic field, in agreement with previous estimates via the Davis-Chandrasekhar-Fermi method, and a ratio of fluctuating to ordered component of the magnetic field of $\sim$ 10\%.Comment: 12 pages, 14 figures, Accepted for publication in MNRA

High-accuracy estimation of magnetic field strength in the interstellar medium from dust polarization

Dust polarization is a powerful tool for studying the magnetic field properties in the interstellar medium (ISM). However, it does not provide a direct measurement of its strength. Different methods havebeen developed which employ both polarization and spectroscopic data in order to infer the field strength. The most widely applied methods have been developed by Davis (1951), Chandrasekhar & Fermi (1953) (DCF), Hildebrand et al. (2009) and Houde et al.(2009) (HH09). They rely on the assumption that isotropic turbulent motions initiate the propagation of Alvf\'en waves. Observations,however, indicate that turbulence in the ISM is anisotropic and non-Alfv\'enic (compressible) modes may be important. Our goal is to develop a new method for estimating the field strength in the ISM, which includes the compressible modes and does not contradict the anisotropic properties of turbulence. We use simple energetics arguments that take into account the compressible modes to estimate the strength of the magnetic field. We derive the following equation: $B_{0}=\sqrt{2 \pi\rho} \delta v /\sqrt{\delta \theta}$, where $\rho$ is the gas density, $\delta v$ is the rms velocity as derived from the spread of emission lines, and $\delta \theta$ is the dispersion of polarization angles. We produce synthetic observations from 3D MHD simulationsand we assess the accuracy of our method by comparing the true field strength with the estimates derived from our equation. We find a mean relative deviation of $17 \%$. The accuracy of our method does not depend on the turbulence properties of the simulated model. In contrast DCF and HH09 systematically overestimate the field strength. HH09 produces accurate results only for simulations with high sonic Mach numbers.Comment: Accepted for publication to Astronomy & Astrophysic

What Molecular Abundances Can Tell Us About The Dynamics Of Star Formation

Molecular clouds are the sites where new stars form. Spectroscopic observations of different molecular species in these clouds can provide invaluable information regarding the dynamical evolution of star forming sites: first, they provide direct dynamical information (velocities as a function of density); second, they reveal the abundance of various molecules, which in turn depends on the chemodynamical evolutionary stage and history of the observed region. However, the connection between theoretical models of cloud dynamics and astronomical molecular spectroscopy is far from straight forward. The chemistry and dynamics of the clouds are interlinked, and various parameters such as the cloud temperature and its initial elemental abundances affect theoretical predictions, resulting in large model degeneracies: radically different dynamical models can often result in similar molecular abundances. In this talk, I will discuss first results from a massive effort undertaken to overcome this problem. By coupling non-equilibrium chemistry with a large array of different dynamical models of molecular cloud evolution, we are looking for these molecular line observables that are least affected by varying parameters and model degeneracies, and can be used to drastically constrain the possible dynamical histories of observed star-forming regions. To this end, we have studied a variety of dynamical models describing the evolution of pre- stellar molecular cloud cores (the initial phase of star formation) that cover the entire spectrum of proposed mechanisms, including pure hydrodynamical collACSe and magnetically mediated collACSe at various levels of importance of the magnetic field in the cloud dynamics. These models have been coupled to a network of chemical reactions that follow the relative abundances for $\sim$100 molecular species, by solving the non- equilibrium chemical reactions for the first time simultaneously with the dynamical equations. I will present highlights from the results of this work, including newly proposed observables with maximal potential for discrimination between different models of cloud evolution and star formation. These results are especially timely as ALMA is able to measure many of these quantities and contribute to the resolution of long-standing questions in star formation, such as the timescale of pre-stellar core evolution, and the relative importance of magnetic field and turbulence in their dynamics

Protostar Formation in Magnetic Molecular Clouds beyond Ion Detachment: III. A Parameter Study

In two previous papers we formulated and solved, for a fiducial set of free parameters, the problem of the formation and evolution of a magnetically supercritical core inside a magnetically subcritical parent cloud. In this paper we present a parameter study to assess the sensitivity of the results (1) to the density at which the equation of state becomes adiabatic; (2) to the initial mass-to-flux ratio of the parent cloud; and (3) to ionization by radioactive decay of different nuclei (40K and 26Al) at high densities (number density > 10^12 particles per cubic cm). We find that (1) the results depend only slightly on the density at which the onset of adiabaticity occurs; (2) memory of the initial mass-to-flux ratio is completely lost at late times, which emphasizes the relevance of this work, idependently of the adopted theory of core formation; and (3) the precise source of radioactive ionization alters the degree of attachment of the electrons to the field lines (at high densities), and the relative importance of ambipolar diffusion and Ohmic dissipation in reducing the magnetic flux of the protostar. The value of the magnetic field at the end of the runs is insensitive to the values of the free parameters and in excellent agreement with meteoritic measurements of the protosolar nebula magnetic field. The magnetic flux problem of star formation is resolved for at least strongly magnetic newborn stars. A complete detachment of the magnetic field from the matter is unlikely. The formation of a "magnetic wall" (with an associated magnetic shock) is independent of the assumed equation of state, although the process is enhanced and accelerated by the formation of a central hydrostatic core.Comment: 17 pages, 14 figures, emulateapj; accepted for publication in the Astrophysical Journa