The earliest phases of star formation-A Herschel key project. The thermal structure of low-mass molecular cloud cores (original) (raw)
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The Earliest Phases of Star Formation (EPoS): a Herschel key project
Astronomy & Astrophysics, 2013
Context. The temperature and density structure of molecular cloud cores are the most important physical quantities that determine the course of the protostellar collapse and the properties of the stars they form. Nevertheless, density profiles often rely either on the simplifying assumption of isothermality or on observationally poorly constrained model temperature profiles. The instruments of the Herschel satellite provide us for the first time with both the spectral coverage and the spatial resolution that is needed to directly measure the dust temperature structure of nearby molecular cloud cores. Aims. With the aim of better constraining the initial physical conditions in molecular cloud cores at the onset of protostellar collapse, in particular of measuring their temperature structure, we initiated the Guaranteed Time Key Project (GTKP) "The Earliest Phases of Star Formation" (EPoS) with the Herschel satellite. This paper gives an overview of the low-mass sources in the EPoS project, the Herschel and complementary ground-based observations, our analysis method, and the initial results of the survey. Methods. We study the thermal dust emission of 12 previously well-characterized, isolated, nearby globules using FIR and submm continuum maps at up to eight wavelengths between 100 µm and 1.2 mm. Our sample contains both globules with starless cores and embedded protostars at different early evolutionary stages. The dust emission maps are used to extract spatially resolved SEDs, which are then fit independently with modified blackbody curves to obtain line-of-sight-averaged dust temperature and column density maps. Results. We find that the thermal structure of all globules (mean mass 7 M ) is dominated by external heating from the interstellar radiation field and moderate shielding by thin extended halos. All globules have warm outer envelopes (14-20 K) and colder dense interiors (8-12 K) with column densities of a few 10 22 cm −2 . The protostars embedded in some of the globules raise the local temperature of the dense cores only within radii out to about 5000 AU, but do not significantly affect the overall thermal balance of the globules. Five out of the six starless cores in the sample are gravitationally bound and approximately thermally stabilized. The starless core in CB 244 is found to be supercritical and is speculated to be on the verge of collapse. For the first time, we can now also include externally heated starless cores in the L smm / L bol vs. T bol diagram and find that T bol < 25 K seems to be a robust criterion to distinguish starless from protostellar cores, including those that only have an embedded very low-luminosity object.
2013
An attempt is made here to revisit structure formation in a proto-stellar cloud during the early phase of evolution. Molecular cloud subjected to a set of various initial conditions in terms of initial temperature and amplitude of azimuthal density perturbation is investigated numerically. Special emphasis remained on the analysis of ring and spiral type instabilities that have shown dependence on certain initial conditions chosen for a rotating solar mass cloud of molecular hydrogen. Generally, a star forming hydrogen gas is considered to be initially at 10K. We have found that a possible oscillation around this typical value can affect the fate of a collapsing cloud in terms of its evolving structural properties leading to proto-star formation. We explored the initial temperature range of cloud between 8K to 12K and compared physical properties of each within the first phase of proto-star formation. We suggest that the spiral structures are more likely to form in strongly perturbe...
2013
An attempt is made here to revisit structure formation in a proto-stellar cloud during the early phase of evolution. Molecular cloud subjected to a set of various initial conditions in terms of initial temperature and amplitude of azimuthal density perturbation is investigated numerically. Special emphasis remained on the analysis of ring and spiral type instabilities that have shown dependence on certain initial conditions chosen for a rotating solar mass cloud of molecular hydrogen. Generally, a star forming hydrogen gas is considered to be initially at 10K. We have found that a possible oscillation around this typical value can affect the fate of a collapsing cloud in terms of its evolving structural properties leading to proto-star formation. We explored the initial temperature range of cloud between 8K to 12K and compared physical properties of each within the first phase of proto-star formation. We suggest that the spiral structures are more likely to form in strongly perturbed molecular cores that initiate their phase of collapse from temperatures below 10K. Whereas, cores with initial temperatures above 10K develop, instead of spiral, a ring type structure which subsequently experiences the fragmentation. A transition from spiral to ring instability can be observed at a typical initial temperature of 10K.
The Herschel view of the on-going star formation in the Vela-C molecular cloud
Astronomy & Astrophysics, 2012
Aims. As part of the Herschel guaranteed time key program 'HOBYS', we present the PACS and SPIRE photometric survey of the star forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Our main objectives are to take a census of the cold sources and to derive their mass distribution down to a few solar masses. Methods. Vela-C has been observed with PACS and SPIRE in parallel mode at five wavelengths between 70 µm and 500 µm over an area of about 3 square degrees. A photometric catalogue has been extracted from the detections in each of the five bands, using a threshold of 5 σ over the local background. Out of this catalogue we have selected a robust sub-sample of 268 sources, of which ∼ 75% are cloud clumps (diameter between 0.05 pc and 0.13 pc) and 25% are cores (diameter between 0.025 pc and 0.05 pc). Their Spectral Energy Distributions (SEDs) have been fitted with a modified black body function. We classify 48 sources as protostellar, based on their detection at 70 µm or at shorther wavelengths, and 218 as starless, because of non-detections at 70 µm. For two further sources, we do not provide a secure classification, but suggest they are Class 0 protostars. Results. From SED fitting we have derived key physical parameters (i.e. mass, temperature, bolometric luminosity). Protostellar sources are in general warmer ( T =12.8 K) and more compact ( diameter =0.040 pc) than starless sources ( T =10.3 K, diameter =0.067 pc). Both these evidences can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further. A virial analysis of the starless sources gives an upper limit of 90% for the sources gravitationally bound and therefore prestellar in nature. A luminosity vs. mass diagram of the two populations shows that protostellar sources are in the early accretion phase, while prestellar sources populate a region of the diagram where mass accretion has not started yet. We fit a power law N(logM) ∝ M −1.1±0.2 to the linear portion of the mass distribution of prestellar sources. This is in between that typical of CO clumps and those of cores in nearby star-forming regions. We interpret this as a result of the inhomogeneity of our sample, which is composed of comparable fractions of clumps and cores.
Astronomy and Astrophysics
Context. Isolated starless cores within molecular clouds can be used as a testbed to investigate the conditions prior to the onset of fragmentation and gravitational proto-stellar collapse. Aims. We aim to determine the distribution of the dust temperature and the density of the starless core B68. Methods. In the framework of the Herschel guaranteed time key programme The earliest phases of star formation (EPoS), we have imaged B68 between 100 and 500 µm. Ancillary data at (sub)millimetre wavelengths, spectral line maps of the 12 CO (2–1) and 13 CO (2–1) transitions as well as a NIR extinction map were added to the analysis. We employed a ray-tracing algorithm to derive the 2D mid-plane dust temperature and volume density distribution without suffering from line-of-sight averaging effects of simple SED fitting procedures. Additional 3D radiative transfer calculations were employed to investigate the connection between the external irradiation and the peculiar crescent shaped morphol...
Present-day star formation: From molecular cloud cores to protostars and protoplanetary disks
Progress of Theoretical and Experimental Physics, 2012
Essential physical processes in the formation of protostars and protoplanetary disks are described. Recent advances in non-ideal magnetohydrodynamics simulations, which cover a huge dynamic range from molecular cloud core density (10 4 /cc) to stellar density (10 22 /cc) in a self-consistent manner, enable us to study the realistic evolution of the magnetic field and rotation of protostars and the dynamics of outflows and jets. First we emphasize the importance of radiative heating and cooling, and describe thermal evolution in a self-gravitationally collapsing cloud. The increased pressure at the center creates the first hydrostatic core, which consists of molecular gas. After the dissociation of molecular hydrogen triggers the second gravitational collapse at the center of the first core, a protostar is quickly formed and the first core gradually transforms into a circumstellar disk that eventually accretes onto the central protostar. The importance of the short-lived first core formed in the early collapsing phase is emphasized in the contexts of driving magnetohydrodynamical bipolar outflows and self-gravitational fragmentation into binary or multiple stars. When the central density becomes sufficiently high (10 12 /cc), ohmic dissipation largely removes the magnetic flux from a collapsing cloud core, and the strongly twisted magnetic field lines are straightened. The magnetic field lines are twisted and amplified again for much higher density (10 16 /cc) where the magnetic field is recoupled with warm gas (∼10 3 K). Finally, protostars at their formation epoch have magnetic fields of 0.1-1 kG, which is comparable to observed values of pre-main-sequence stars. A substantially reduced magnetic flux at the center results in passively wound-up magnetic field lines just after the formation of a protostar. This is followed by driving of a fast bipolar jet along the rotation axis by the resultant magnetic pressure due to excessive winding. Strong collimation of the jet is due to the hoop stress of piled-up toroidal field lines. The angular momentum in a collapsing cloud is removed by magnetohydrodynamical effects such as magnetic braking and driving of outflows and jets. The rotation velocity of the protostar tends to be on the order of break-up speed at its formation epoch, and thus, a further removal mechanism for the angular momentum, such as through the interactions between the protostar, disk, jets, and winds, should be important in its long-term evolution. The circumstellar disk is born in the "dead zone," a region that is decoupled from the magnetic field. The outer radius of the disk increases with that of the dead zone during accretion from the envelope of the molecular cloud core. A rapid increase in the disk size occurs after depletion of the envelope. The circumstellar disks remain massive in their formation phase, and are subject to gravitational instability, even at 10 AU from the central stars. The further long-term evolution of massive disks is also described. This may provide an improved description for the realistic initial condition and environments for planet formation in gaseous protoplanetary disks.
The formation and evolution of prestellar cores
Structure Formation in Astrophysics, 2009
Improving our understanding of the initial conditions and earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We review the properties of low-mass dense cores as derived from recent millimeter/submillimeter observations of nearby molecular clouds and discuss them in the context of various contemporary scenarios for cloud core formation and evolution. None of the extreme scenarios can explain all observations. Pure laminar ambipolar diffusion has relatively long growth times for typical ionization levels and has difficulty satisfying core lifetime constraints. Purely hydrodynamic pictures have trouble accounting for the inefficiency of core formation and the detailed velocity structure of individual cores. A possible favorable scenario is a mixed model involving gravitational fragmentation of turbulent molecular clouds close to magnetic criticality. The evolution of the magnetic field and angular momentum in individual cloud cores after the onset of gravitational collapse is also discussed. In particular, we stress the importance of radiationmagnetohydrodynamical processes and resistive MHD effects during the protostellar phase. We also emphasize the role of the formation of the short-lived first (protostellar) core in providing a chance for sub-fragmentation into binary systems and triggering MHD outflows. Future submillimeter facilities such as Herschel and ALMA will soon provide major new observational constraints in this field. On the theoretical side, an important challenge for the future will be to link the formation of molecular clouds and prestellar cores in a coherent picture.
Protostellar collapse models of prolate molecular cloud cores
Astronomy & Astrophysics, 2001
The continued detection of binary systems among pre-main-sequence stars suggests that fragmentation is a very frequent process during the early stages of star formation. However, the fragmentation hypothesis rests only upon the results of three-dimensional hydrodynamics code calculations. The validity of isothermal fragmentation calculations was questioned by the results of Truelove et al. (1997), and more recently, of Boss et al. (2000), who found, working at very high spatial resolution, that a particular Gaussian cloud model collapsed isothermally to form a singular filament rather than a binary or quadruple protostellar system as predicted by previous calculations. Sufficiently high spatial resolution is necessary to resolve the Jeans length and hence avoid artificial fragmentation in isothermal collapse calculations. Here we use an adaptive, spherical-coordinate hydrodynamics code based on the "zooming" coordinates to investigate the isothermal collapse of centrally condensed (Gaussian), prolate (2:1 axial ratio) cloud core models, with thermal energy α ≈ 0.22 and varied rotational energy (0.246 ≤ β ≤ 0.00025), to discern whether they will still undergo fragmentation into a protostellar binary system, as found in most previous prolate cloud collapse calculations, or condense all the way into a thin filament, as suggested by the linear analysis of Inutsuka & Miyama (1992) and the findings of Truelove et al. and Boss et al. for the spherical, Gaussian cloud model. The prolate clouds all collapsed self-similarly to produce an intermediate barlike core, which then shrank indefinetely into a singular filament without fragmenting. Collapse of the bar into a thin filament also occurred self-similarly, with the forming filaments being much longer than the Jeans length. Since the filaments form at maximum densities that are typical of the transition from the isothermal to the nonisothermal phase, gradual heating may retard the collapse and allow fragmentation of the filament into a binary or multiple protostellar core, as required to explain the high frequency of binary stars.
Global collapse of molecular clouds as a formation mechanism for the most massive stars
2013
The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(±800) M ⊙ infrared dark cloud SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545( +770 −385 ) M ⊙ contained within a source diameter of 0.05 pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V in f = 0.7(±0.2) km/s, and a total mass infall rateṀ in f ≃ 2.5(±1.0) × 10 −3 M ⊙ yr −1 towards the central pc-size region of SDC335. This infall rate brings 750(±300) M ⊙ of gas to the centre of the cloud per free-fall time (t f f = 3 × 10 5 yr). This is enough to double the mass already present in the central pc-size region in 3.5 +2.2 −1.0 × t f f . These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well.