Research performed in the 1950s and 1960s by Leon Mestel on the roles of magnetic fields in star ... more Research performed in the 1950s and 1960s by Leon Mestel on the roles of magnetic fields in star formation established the framework within which he and other key figures have conducted subsequent investigations on the subject. This short tribute to Leon contains a brief summary of some, but not all, of his ground breaking contributions in the area. It also mentions of some of the relevant problems that have received attention in the last few years. The coverage is not comprehensive, and the authors have drawn on their own results more and touched more briefly on those of others than they would in a normal review. Theirs is a personal contribution to the issue honouring Leon, one of the truly great gentlemen, wits, and most insightful of astrophysicists.
Continuum emission at 220 and 355 GHz from the prototype Class 0 source VLA 1623 has been detecte... more Continuum emission at 220 and 355 GHz from the prototype Class 0 source VLA 1623 has been detected using the JCMT-CSO interferometer. Gaussian fits to the data place an upper limit of 70 AU on the half-width half-maximum radius of the emission, which implies an upper limit of ∼ 175 AU for the cutoff radius of the circumstellar disk in the system. In the context of existing collapse models, this disk could be magnetically supported on the largest scales and have an age of ∼ 6 × 10 4 yr, consistent with previous suggestions that Class 0 sources are quite young. The innermost region of the disk within ∼ 6 AU is likely to be in centrifugal support, which is likely large enough to provide a drive for the outflow according to current theoretical models. Alternatively, if 175 AU corresponds to the centrifugal radius of the disk, the age of the system is ∼ 2 × 10 5 yr, closer to age estimates for Class I sources.
Monthly Notices of the Royal Astronomical Society, 2000
In Paper I (this issue), we constructed models of filamentary molecular clouds that are truncated... more In Paper I (this issue), we constructed models of filamentary molecular clouds that are truncated by a realistic external pressure and contain a rather general helical magnetic field. We address the stability of our models to gravitational fragmentation and axisymmetric magnetohydrodynamically driven instabilities. By calculating the dominant modes of axisymmetric instability, we determine the dominant length-scales and growth rates for fragmentation. We find that the role of pressure truncation is to decrease the growth rate of gravitational instabilities by decreasing the self-gravitating mass per unit length. Purely poloidal and toroidal fields also help to stabilize filamentary clouds against fragmentation. The overall effect of helical fields is to stabilize gravity-driven modes, so that the growth rates are significantly reduced below what is expected for unmagnetized clouds. However, magnetohydrodynamic (MHD)`sausage' instabilities are triggered in models in which the toroidal flux to mass ratio exceeds the poloidal flux to mass ratio by more than a factor of ,2. We find that observed filaments appear to lie in a physical regime where the growth rates of both gravitational fragmentation and axisymmetric MHD-driven modes are at a minimum.
Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that fo... more Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars (M ≳ 8 M ⊙) are still not well understood. A growing number of case studies have found massive young stellar objects (MYSOs) to harbour disk-like structures, painting a picture that the formation of high-mass stars may proceed through disk accretion, similar to that of lower-mass stars. However, the properties of such structures have yet to be uniformly and systematically characterised. Aims. The aim of this work is to uniformly study the kinematic properties of a large sample of MYSOs and characterise the stability of possible circumstellar disks against gravitational fragmentation. Methods. We have undertaken a large observational programme (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous (L > 10 4 L ⊙) protostellar objects in the 1.37 mm wavelength regime in both continuum and spectral line emission, reaching 0.4 ′′ resolution (800 au at 2 kpc). Results. We present the gas kinematics of the full sample and detect dense gas emission surrounding 15 regions within the CORE sample. Using the dense gas tracer CH 3 CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. The extent of the CH 3 CN emission tracing the disk candidates varies from 1800 to 8500 au. Analysing the free-fall to rotational timescales, we find that the sources are rotationally supported. The rotation profiles of some disk candidates are well described by differential rotation while for others the profiles are poorly resolved. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of ∼10-25 M ⊙. Modelling the level population of CH 3 CN (12 K −11 K) K = 0−6 lines, we present temperature maps and find median temperature in the range 70-210 K with a diversity in distributions. Radial profiles of the specific angular momentum (j) for the best disk candidates span a range of 1-2 orders of magnitude, on average ∼10 −3 km s −1 pc, and they follow j ∝ r 1.7 , which is consistent with a poorly resolved rotating and infalling envelope-disk model. Studying the Toomre stability of the disk candidates, we find almost all (11 out of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations, as a result of their high disk to stellar mass ratio. In particular, disks with masses greater than ∼10-20% of the mass of their host (proto)stars are Toomre unstable, and more luminous YSOs tend to have disks that are more massive compared to their host star and hence more prone to fragmentation. Conclusions. In this work, we show that most disk structures around high-mass YSOs are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.
In Paper I (Fiege & Pudritz, 1999), we constructed models of filamentary molecular clouds that ar... more In Paper I (Fiege & Pudritz, 1999), we constructed models of filamentary molecular clouds that are truncated by a realistic external pressure and contain a rather general helical magnetic field. We address the stability of our models to gravitational fragmentation and axisymmetric MHD-driven instabilities. By calculating the dominant modes of axisymmetric instability, we determine the dominant length scales and growth rates for fragmentation. We find that the role of pressure truncation is to decrease the growth rate of gravitational instabilities by decreasing the self-gravitating mass per unit length. Purely poloidal and toroidal fields also help to stabilize filamentary clouds against fragmentation. The overall effect of helical fields is to stabilize gravity-driven modes, so that the growth rates are significantly reduced below what is expected for unmagnetized clouds. However, MHD "sausage" instabilities are triggered in models whose toroidal flux to mass ratio exceeds the poloidal flux to mass ratio by more than a factor of ∼ 2. We find that observed filaments appear to lie in a physical regime where the growth rates of both gravitational fragmentation and axisymmetric MHD-driven modes are at a minimum.
Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbo... more Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbonaceous chondrites. The exogenous delivery of these organic molecules onto the Hadean Earth could have sparked the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. Here, we investigate the formation of the RNA and DNA nucleobases adenine, uracil, cytosine, guanine, and thymine inside parent body planetesimals of carbonaceous chondrites. An up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model is used to calculate the nucleobase concentrations. Different from the previous study (Pearce & Pudritz 2016), we assume initial volatile concentrations more appropriate for the formation zone of carbonaceous chondrite parent bodies. This represents more accurately cosmochemical findings that these bodies have formed inside the inner, ∼ 2-5 au, warm region of the solar system. Due to these improvements, our model represents the concentrations of adenine and guanine measured in carbonaceous chondrites. Our model did not reproduce per se the measurements of uracil, cytosine, and thymine in these meteorites. This can be explained by transformation reactions between nucleobases and potential decomposition of thymine. The synthesis of prebiotic organic matter in carbonaceous asteroids could be well explained by a combination of i) radiogenic heating, ii) aqueous chemistry involving a few key processes at a specific range of radii inside planetesimals where water can exist in the liquid phase, and iii) a reduced initial volatile content (H 2 , CO, HCN, CH 2 O) of the protoplanetary disk material in the parent body region compared to the outer region of comets.
Context. This study is part of the CORE project, an IRAM/NOEMA large program consisting of observ... more Context. This study is part of the CORE project, an IRAM/NOEMA large program consisting of observations of the millimeter continuum and molecular line emission towards 20 selected high-mass star-forming regions. The goal of the program is to search for circumstellar accretion disks, study the fragmentation process of molecular clumps, and investigate the chemical composition of the gas in these regions. Aims. We focus on IRAS 23385+6053, which is believed to be the least-evolved source of the CORE sample. This object is characterized by a compact molecular clump that is IR-dark shortward of 24 µm and is surrounded by a stellar cluster detected in the near-IR. Our aim is to study the structure and velocity field of the clump. Methods. Observations were performed at ∼1.4 mm and employed three configurations of NOEMA and additional single-dish maps, merged with the interferometric data to recover the extended emission. Our correlator setup covered a number of lines from well-known hot core tracers and a few outflow tracers. The angular (∼0. 45-0. 9) and spectral (0.5 km s −1) resolutions were sufficient to resolve the clump in IRAS 23385+6053 and investigate the existence of large-scale motions due to rotation, infall, or expansion. Results. We find that the clump splits into six distinct cores when observed at sub-arcsecond resolution. These are identified through their 1.4 mm continuum and molecular line emission. We produce maps of the velocity, line width, and rotational temperature from the methanol and methyl cyanide lines, which allow us to investigate the cores and reveal a velocity and temperature gradient in the most massive core. We also find evidence of a bipolar outflow, possibly powered by a low-mass star. Conclusions. We present the tentative detection of a circumstellar self-gravitating disk lying in the most massive core and powering a large-scale outflow previously known in the literature. In our scenario, the star powering the flow is responsible for most of the luminosity of IRAS 23385+6053 (∼3000 L). The other cores, albeit with masses below the corresponding virial masses, appear to be accreting material from their molecular surroundings and are possibly collapsing or on the verge of collapse. We conclude that we are observing a sample of star-forming cores that is bound to turn into a cluster of massive stars.
HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We cal... more HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50-400 K. We present 15 new reaction rate coefficients with no previous known value. 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN − −→ HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 − −→ HCN + H2, to be inefficient, and suggest the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.
Monthly Notices of the Royal Astronomical Society: Letters, May 25, 2018
Observations show that galaxies and their interstellar media are pervaded by strong magnetic fiel... more Observations show that galaxies and their interstellar media are pervaded by strong magnetic fields with energies in the diffuse component being at least comparable to the thermal and even as large or larger than the turbulent energy. Such strong magnetic fields prevent the formation of stars because patches of the interstellar medium are magnetically subcritical. Here we present the results from global numerical simulations of strongly magnetized and self-gravitating galactic discs, which show that the buoyancy of the magnetic field due to the Parker instability leads at first to the formation of giant filamentary regions. These filamentary structures become gravitationally unstable and fragment into ∼10 5 M clouds that attract kpc long coherent filamentary flows that build them into giant molecular clouds (GMCs). Our results thus provide a solution to the long-standing problem of how the transition from sub-to supercritical regions in the interstellar medium proceeds.
Monthly Notices of the Royal Astronomical Society, Sep 10, 2019
Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impa... more Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impact on gas dynamics and star formation in galaxies remains controversial. We use a suite of global magnetohydrodynamical simulations of isolated disc galaxies to study the influence of magnetic fields on the diffuse and dense gas in the discs. We find that the magnetic field acts in multiple ways. Stronger magnetised discs fragment earlier due to the shorter growth time of the Parker instability. Due to the Parker instability in the magnetised discs we also find cold (T < 50 K) and dense (n ∼ 10 3 − 10 4 cm −3) gas several hundred pc above/below the midplane without any form of stellar feedback. In addition, magnetic fields change the fragmentation pattern. While in the hydrodynamical case, the disc breaks up into ring-like structures, magnetised discs show the formation of filamentary entities that extent both in the azimuthal and radial direction. These kpc scale filaments become magnetically (super-)critical very quickly and allow for the rapid formation of massive giant molecular clouds. Our simulations suggest that major differences in the behaviour of star formation-due to a varying magnetisation-in galaxies could arise.
Appendix B: Critical Field Strengths for Stability. 99 4 VERTICAL AND AZIMUTHAL MAGNETIC FIELDS 1... more Appendix B: Critical Field Strengths for Stability. 99 4 VERTICAL AND AZIMUTHAL MAGNETIC FIELDS 110 4.1 Introduction ... 111 4.2 The Equilibrium 114 4.2.1 Basic equations. 114 4.2.2 Special cases 116 4.3 The Perturbations 117 4.3.1 The perturbation equations 117 4.3.2 The boundary conditions li9 4.4 Results: Constant Vertical Field 4.4.1 The case of a = b. .. . 4.4.2 Critic'Ll stability curves 12:3 4.4.3 The large-field instability 4.4.4 Free boundaries.. .. . 4.4.5 The general case: a i' b 4.4.6 The effect of simulated vertical boundaries 4.5 Nonconstant Vertical Field
To understand the role that planet formation history has on the observable atmospheric carbon-to-... more To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets, their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a "main sequence" between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O; planets of higher solid accretion fraction have lower C/O. Hot Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that hot-Jupiter formation typically begins near the water ice line. Lower mass hot Neptunes are observed to be much more carbon rich (with 0.33 C/O 1) than is found in our models (C/O ∼ 10 −2), and suggest that some form of chemical processing may affect their observed C/O over the few billion years between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.
Monthly Notices of the Royal Astronomical Society, Oct 11, 1999
We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflo... more We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium. We explore the dependence of the resulting jet characteristics upon the mass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration. We show that the nature of the resulting highly collimated, jet-like outflows (steady or episodic) is determined by the mass load of the disc wind. The mass load controls the interplay between the collimating effects of the toroidal field and the kinetic energy density in the outflow. In this regard, we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity N B 2 f a4prv 2 p which compares the speed for a toroidal Alfve Ân wave to cross the diameter of the jet, with the flow speed v p along the jet. This quantity decreases with increasing load. For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outflow becomes episodic. These effects are qualitatively independent of the initial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.
This article summarizes recent theoretical, numerical and observational progress on the nature of... more This article summarizes recent theoretical, numerical and observational progress on the nature of jets and outflows in regions of star formation. The emphasis is placed on the role of hydromagnetic phenonmena in understanding these new results. The confluence of sophisticated 3D numerical MHD simulations and the recent observations of rotating jets makes it possible, for the first time, to rigorously test jet models.
The discovery of over 160 extrasolar planets, many with very unusual properties, has driven a ren... more The discovery of over 160 extrasolar planets, many with very unusual properties, has driven a renaissance in the study of planet formation. It is likely that Jovian and supra-Jovian planets formed at large distances, migrated towards their central stars, and yet somehow managed to stop short of plunging in. The growing theoretical and observational study of protostellar disks is allowing us to better probe the mechanism(s) of planet formation and migration through disk-gas interactions. In this talk, we explore new insights in mechanisms for determing planetary masses that arise from the presence of turbulence-free, so called dead-zones in protostellar disks. Such regions should be present on scales of up to 15 AU in most disks and should have profound effects on the migration of both terrestrial and Jovian planets - in effect - saving planetary systems. We also explore some obervational consequences of such ideas for observing programmes that can be implemented at the planned new large-scale ground-based facilities; TMT and SKA.
To understand the role that planet formation history has on the observable atmospheric carbon-to-... more To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets, their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a "main sequence" between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O; planets of higher solid accretion fraction have lower C/O. Hot Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that hot-Jupiter formation typically begins near the water ice line. Lower mass hot Neptunes are observed to be much more carbon rich (with 0.33 C/O 1) than is found in our models (C/O ∼ 10 −2), and suggest that some form of chemical processing may affect their observed C/O over the few billion years between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.
The coming decade promises to see the most progression to date in understanding the formation and... more The coming decade promises to see the most progression to date in understanding the formation and early evolution of extrasolar planetary systems. The first five years will see the culmination of more than a decade's work to get planned surveys in the far-infrared and submillimetre on the sky through the Herschel Space Observatory and the James Clerk Maxwell Telescope. Most importantly, the completion of ALMA construction in 2013 will provide the critical instrumentation to resolve and understand the dynamics, chemistry and evolution of circumstellar disks in star forming regions and the substructure of debris disks around main sequence stars, counterparts to our own outer solar system and Kuiper Belt. To make the most effective use of the opportunities for advancing disk studies in the coming decade, Canada must foster a vigorous, well populated community of researchers to take advantage of our (essentially) unlimited access to ALMA and EVLA; complete the JCMT Legacy Survey, critical for the detection of disk hosts which will be the ALMA and EVLA targets; work to provide the means to build research teams across Canada, including at NRC; encourage computing consortia to meet the challenge of disk theory and modeling simulations; and pursue Band 1 development for ALMA. Subject headings:
Research performed in the 1950s and 1960s by Leon Mestel on the roles of magnetic fields in star ... more Research performed in the 1950s and 1960s by Leon Mestel on the roles of magnetic fields in star formation established the framework within which he and other key figures have conducted subsequent investigations on the subject. This short tribute to Leon contains a brief summary of some, but not all, of his ground breaking contributions in the area. It also mentions of some of the relevant problems that have received attention in the last few years. The coverage is not comprehensive, and the authors have drawn on their own results more and touched more briefly on those of others than they would in a normal review. Theirs is a personal contribution to the issue honouring Leon, one of the truly great gentlemen, wits, and most insightful of astrophysicists.
Continuum emission at 220 and 355 GHz from the prototype Class 0 source VLA 1623 has been detecte... more Continuum emission at 220 and 355 GHz from the prototype Class 0 source VLA 1623 has been detected using the JCMT-CSO interferometer. Gaussian fits to the data place an upper limit of 70 AU on the half-width half-maximum radius of the emission, which implies an upper limit of ∼ 175 AU for the cutoff radius of the circumstellar disk in the system. In the context of existing collapse models, this disk could be magnetically supported on the largest scales and have an age of ∼ 6 × 10 4 yr, consistent with previous suggestions that Class 0 sources are quite young. The innermost region of the disk within ∼ 6 AU is likely to be in centrifugal support, which is likely large enough to provide a drive for the outflow according to current theoretical models. Alternatively, if 175 AU corresponds to the centrifugal radius of the disk, the age of the system is ∼ 2 × 10 5 yr, closer to age estimates for Class I sources.
Monthly Notices of the Royal Astronomical Society, 2000
In Paper I (this issue), we constructed models of filamentary molecular clouds that are truncated... more In Paper I (this issue), we constructed models of filamentary molecular clouds that are truncated by a realistic external pressure and contain a rather general helical magnetic field. We address the stability of our models to gravitational fragmentation and axisymmetric magnetohydrodynamically driven instabilities. By calculating the dominant modes of axisymmetric instability, we determine the dominant length-scales and growth rates for fragmentation. We find that the role of pressure truncation is to decrease the growth rate of gravitational instabilities by decreasing the self-gravitating mass per unit length. Purely poloidal and toroidal fields also help to stabilize filamentary clouds against fragmentation. The overall effect of helical fields is to stabilize gravity-driven modes, so that the growth rates are significantly reduced below what is expected for unmagnetized clouds. However, magnetohydrodynamic (MHD)`sausage' instabilities are triggered in models in which the toroidal flux to mass ratio exceeds the poloidal flux to mass ratio by more than a factor of ,2. We find that observed filaments appear to lie in a physical regime where the growth rates of both gravitational fragmentation and axisymmetric MHD-driven modes are at a minimum.
Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that fo... more Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars (M ≳ 8 M ⊙) are still not well understood. A growing number of case studies have found massive young stellar objects (MYSOs) to harbour disk-like structures, painting a picture that the formation of high-mass stars may proceed through disk accretion, similar to that of lower-mass stars. However, the properties of such structures have yet to be uniformly and systematically characterised. Aims. The aim of this work is to uniformly study the kinematic properties of a large sample of MYSOs and characterise the stability of possible circumstellar disks against gravitational fragmentation. Methods. We have undertaken a large observational programme (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous (L > 10 4 L ⊙) protostellar objects in the 1.37 mm wavelength regime in both continuum and spectral line emission, reaching 0.4 ′′ resolution (800 au at 2 kpc). Results. We present the gas kinematics of the full sample and detect dense gas emission surrounding 15 regions within the CORE sample. Using the dense gas tracer CH 3 CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. The extent of the CH 3 CN emission tracing the disk candidates varies from 1800 to 8500 au. Analysing the free-fall to rotational timescales, we find that the sources are rotationally supported. The rotation profiles of some disk candidates are well described by differential rotation while for others the profiles are poorly resolved. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of ∼10-25 M ⊙. Modelling the level population of CH 3 CN (12 K −11 K) K = 0−6 lines, we present temperature maps and find median temperature in the range 70-210 K with a diversity in distributions. Radial profiles of the specific angular momentum (j) for the best disk candidates span a range of 1-2 orders of magnitude, on average ∼10 −3 km s −1 pc, and they follow j ∝ r 1.7 , which is consistent with a poorly resolved rotating and infalling envelope-disk model. Studying the Toomre stability of the disk candidates, we find almost all (11 out of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations, as a result of their high disk to stellar mass ratio. In particular, disks with masses greater than ∼10-20% of the mass of their host (proto)stars are Toomre unstable, and more luminous YSOs tend to have disks that are more massive compared to their host star and hence more prone to fragmentation. Conclusions. In this work, we show that most disk structures around high-mass YSOs are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.
In Paper I (Fiege & Pudritz, 1999), we constructed models of filamentary molecular clouds that ar... more In Paper I (Fiege & Pudritz, 1999), we constructed models of filamentary molecular clouds that are truncated by a realistic external pressure and contain a rather general helical magnetic field. We address the stability of our models to gravitational fragmentation and axisymmetric MHD-driven instabilities. By calculating the dominant modes of axisymmetric instability, we determine the dominant length scales and growth rates for fragmentation. We find that the role of pressure truncation is to decrease the growth rate of gravitational instabilities by decreasing the self-gravitating mass per unit length. Purely poloidal and toroidal fields also help to stabilize filamentary clouds against fragmentation. The overall effect of helical fields is to stabilize gravity-driven modes, so that the growth rates are significantly reduced below what is expected for unmagnetized clouds. However, MHD "sausage" instabilities are triggered in models whose toroidal flux to mass ratio exceeds the poloidal flux to mass ratio by more than a factor of ∼ 2. We find that observed filaments appear to lie in a physical regime where the growth rates of both gravitational fragmentation and axisymmetric MHD-driven modes are at a minimum.
Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbo... more Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbonaceous chondrites. The exogenous delivery of these organic molecules onto the Hadean Earth could have sparked the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. Here, we investigate the formation of the RNA and DNA nucleobases adenine, uracil, cytosine, guanine, and thymine inside parent body planetesimals of carbonaceous chondrites. An up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model is used to calculate the nucleobase concentrations. Different from the previous study (Pearce & Pudritz 2016), we assume initial volatile concentrations more appropriate for the formation zone of carbonaceous chondrite parent bodies. This represents more accurately cosmochemical findings that these bodies have formed inside the inner, ∼ 2-5 au, warm region of the solar system. Due to these improvements, our model represents the concentrations of adenine and guanine measured in carbonaceous chondrites. Our model did not reproduce per se the measurements of uracil, cytosine, and thymine in these meteorites. This can be explained by transformation reactions between nucleobases and potential decomposition of thymine. The synthesis of prebiotic organic matter in carbonaceous asteroids could be well explained by a combination of i) radiogenic heating, ii) aqueous chemistry involving a few key processes at a specific range of radii inside planetesimals where water can exist in the liquid phase, and iii) a reduced initial volatile content (H 2 , CO, HCN, CH 2 O) of the protoplanetary disk material in the parent body region compared to the outer region of comets.
Context. This study is part of the CORE project, an IRAM/NOEMA large program consisting of observ... more Context. This study is part of the CORE project, an IRAM/NOEMA large program consisting of observations of the millimeter continuum and molecular line emission towards 20 selected high-mass star-forming regions. The goal of the program is to search for circumstellar accretion disks, study the fragmentation process of molecular clumps, and investigate the chemical composition of the gas in these regions. Aims. We focus on IRAS 23385+6053, which is believed to be the least-evolved source of the CORE sample. This object is characterized by a compact molecular clump that is IR-dark shortward of 24 µm and is surrounded by a stellar cluster detected in the near-IR. Our aim is to study the structure and velocity field of the clump. Methods. Observations were performed at ∼1.4 mm and employed three configurations of NOEMA and additional single-dish maps, merged with the interferometric data to recover the extended emission. Our correlator setup covered a number of lines from well-known hot core tracers and a few outflow tracers. The angular (∼0. 45-0. 9) and spectral (0.5 km s −1) resolutions were sufficient to resolve the clump in IRAS 23385+6053 and investigate the existence of large-scale motions due to rotation, infall, or expansion. Results. We find that the clump splits into six distinct cores when observed at sub-arcsecond resolution. These are identified through their 1.4 mm continuum and molecular line emission. We produce maps of the velocity, line width, and rotational temperature from the methanol and methyl cyanide lines, which allow us to investigate the cores and reveal a velocity and temperature gradient in the most massive core. We also find evidence of a bipolar outflow, possibly powered by a low-mass star. Conclusions. We present the tentative detection of a circumstellar self-gravitating disk lying in the most massive core and powering a large-scale outflow previously known in the literature. In our scenario, the star powering the flow is responsible for most of the luminosity of IRAS 23385+6053 (∼3000 L). The other cores, albeit with masses below the corresponding virial masses, appear to be accreting material from their molecular surroundings and are possibly collapsing or on the verge of collapse. We conclude that we are observing a sample of star-forming cores that is bound to turn into a cluster of massive stars.
HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We cal... more HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50-400 K. We present 15 new reaction rate coefficients with no previous known value. 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN − −→ HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 − −→ HCN + H2, to be inefficient, and suggest the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.
Monthly Notices of the Royal Astronomical Society: Letters, May 25, 2018
Observations show that galaxies and their interstellar media are pervaded by strong magnetic fiel... more Observations show that galaxies and their interstellar media are pervaded by strong magnetic fields with energies in the diffuse component being at least comparable to the thermal and even as large or larger than the turbulent energy. Such strong magnetic fields prevent the formation of stars because patches of the interstellar medium are magnetically subcritical. Here we present the results from global numerical simulations of strongly magnetized and self-gravitating galactic discs, which show that the buoyancy of the magnetic field due to the Parker instability leads at first to the formation of giant filamentary regions. These filamentary structures become gravitationally unstable and fragment into ∼10 5 M clouds that attract kpc long coherent filamentary flows that build them into giant molecular clouds (GMCs). Our results thus provide a solution to the long-standing problem of how the transition from sub-to supercritical regions in the interstellar medium proceeds.
Monthly Notices of the Royal Astronomical Society, Sep 10, 2019
Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impa... more Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impact on gas dynamics and star formation in galaxies remains controversial. We use a suite of global magnetohydrodynamical simulations of isolated disc galaxies to study the influence of magnetic fields on the diffuse and dense gas in the discs. We find that the magnetic field acts in multiple ways. Stronger magnetised discs fragment earlier due to the shorter growth time of the Parker instability. Due to the Parker instability in the magnetised discs we also find cold (T < 50 K) and dense (n ∼ 10 3 − 10 4 cm −3) gas several hundred pc above/below the midplane without any form of stellar feedback. In addition, magnetic fields change the fragmentation pattern. While in the hydrodynamical case, the disc breaks up into ring-like structures, magnetised discs show the formation of filamentary entities that extent both in the azimuthal and radial direction. These kpc scale filaments become magnetically (super-)critical very quickly and allow for the rapid formation of massive giant molecular clouds. Our simulations suggest that major differences in the behaviour of star formation-due to a varying magnetisation-in galaxies could arise.
Appendix B: Critical Field Strengths for Stability. 99 4 VERTICAL AND AZIMUTHAL MAGNETIC FIELDS 1... more Appendix B: Critical Field Strengths for Stability. 99 4 VERTICAL AND AZIMUTHAL MAGNETIC FIELDS 110 4.1 Introduction ... 111 4.2 The Equilibrium 114 4.2.1 Basic equations. 114 4.2.2 Special cases 116 4.3 The Perturbations 117 4.3.1 The perturbation equations 117 4.3.2 The boundary conditions li9 4.4 Results: Constant Vertical Field 4.4.1 The case of a = b. .. . 4.4.2 Critic'Ll stability curves 12:3 4.4.3 The large-field instability 4.4.4 Free boundaries.. .. . 4.4.5 The general case: a i' b 4.4.6 The effect of simulated vertical boundaries 4.5 Nonconstant Vertical Field
To understand the role that planet formation history has on the observable atmospheric carbon-to-... more To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets, their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a "main sequence" between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O; planets of higher solid accretion fraction have lower C/O. Hot Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that hot-Jupiter formation typically begins near the water ice line. Lower mass hot Neptunes are observed to be much more carbon rich (with 0.33 C/O 1) than is found in our models (C/O ∼ 10 −2), and suggest that some form of chemical processing may affect their observed C/O over the few billion years between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.
Monthly Notices of the Royal Astronomical Society, Oct 11, 1999
We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflo... more We present 2.5D time-dependent simulations of the non-linear evolution of non-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium. We explore the dependence of the resulting jet characteristics upon the mass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration. We show that the nature of the resulting highly collimated, jet-like outflows (steady or episodic) is determined by the mass load of the disc wind. The mass load controls the interplay between the collimating effects of the toroidal field and the kinetic energy density in the outflow. In this regard, we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity N B 2 f a4prv 2 p which compares the speed for a toroidal Alfve Ân wave to cross the diameter of the jet, with the flow speed v p along the jet. This quantity decreases with increasing load. For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outflow becomes episodic. These effects are qualitatively independent of the initial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.
This article summarizes recent theoretical, numerical and observational progress on the nature of... more This article summarizes recent theoretical, numerical and observational progress on the nature of jets and outflows in regions of star formation. The emphasis is placed on the role of hydromagnetic phenonmena in understanding these new results. The confluence of sophisticated 3D numerical MHD simulations and the recent observations of rotating jets makes it possible, for the first time, to rigorously test jet models.
The discovery of over 160 extrasolar planets, many with very unusual properties, has driven a ren... more The discovery of over 160 extrasolar planets, many with very unusual properties, has driven a renaissance in the study of planet formation. It is likely that Jovian and supra-Jovian planets formed at large distances, migrated towards their central stars, and yet somehow managed to stop short of plunging in. The growing theoretical and observational study of protostellar disks is allowing us to better probe the mechanism(s) of planet formation and migration through disk-gas interactions. In this talk, we explore new insights in mechanisms for determing planetary masses that arise from the presence of turbulence-free, so called dead-zones in protostellar disks. Such regions should be present on scales of up to 15 AU in most disks and should have profound effects on the migration of both terrestrial and Jovian planets - in effect - saving planetary systems. We also explore some obervational consequences of such ideas for observing programmes that can be implemented at the planned new large-scale ground-based facilities; TMT and SKA.
To understand the role that planet formation history has on the observable atmospheric carbon-to-... more To understand the role that planet formation history has on the observable atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of astrochemically evolving protoplanetary disks. Based on the parameters used in a pre-computed population of growing planets, their combination allows us to trace the molecular abundances of the gas that is being collected into planetary atmospheres. We include atmospheric pollution of incoming (icy) planetesimals as well as the effect of refractory carbon erosion noted to exist in our own solar system. We find that the carbon and oxygen content of Neptune-mass planets are determined primarily through solid accretion and result in more oxygen-rich (by roughly two orders of magnitude) atmospheres than hot Jupiters, whose C/O are primarily determined by gas accretion. Generally we find a "main sequence" between the fraction of planetary mass accreted through solid accretion and the resulting atmospheric C/O; planets of higher solid accretion fraction have lower C/O. Hot Jupiters whose atmospheres have been chemically characterized agree well with our population of planets, and our results suggest that hot-Jupiter formation typically begins near the water ice line. Lower mass hot Neptunes are observed to be much more carbon rich (with 0.33 C/O 1) than is found in our models (C/O ∼ 10 −2), and suggest that some form of chemical processing may affect their observed C/O over the few billion years between formation and observation. Our population reproduces the general mass-metallicity trend of the solar system and qualitatively reproduces the C/O metallicity anti-correlation that has been inferred for the population of characterized exoplanetary atmospheres.
The coming decade promises to see the most progression to date in understanding the formation and... more The coming decade promises to see the most progression to date in understanding the formation and early evolution of extrasolar planetary systems. The first five years will see the culmination of more than a decade's work to get planned surveys in the far-infrared and submillimetre on the sky through the Herschel Space Observatory and the James Clerk Maxwell Telescope. Most importantly, the completion of ALMA construction in 2013 will provide the critical instrumentation to resolve and understand the dynamics, chemistry and evolution of circumstellar disks in star forming regions and the substructure of debris disks around main sequence stars, counterparts to our own outer solar system and Kuiper Belt. To make the most effective use of the opportunities for advancing disk studies in the coming decade, Canada must foster a vigorous, well populated community of researchers to take advantage of our (essentially) unlimited access to ALMA and EVLA; complete the JCMT Legacy Survey, critical for the detection of disk hosts which will be the ALMA and EVLA targets; work to provide the means to build research teams across Canada, including at NRC; encourage computing consortia to meet the challenge of disk theory and modeling simulations; and pursue Band 1 development for ALMA. Subject headings:
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Papers by Ralph Pudritz