Filamentary Molecular Clouds Research Articles

Helical fields and filamentary molecular clouds -- I

We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds. We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing ‘isothermal’ filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state. Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r−1.8 to r−2, while logatropes have density profiles that range from r−1 to r−1.8. We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.

What Determines the Typical Mass of Dense Coresin Quiescent, Nonmagnetized Molecular Clouds?

By means of one-dimensional simulations, we study the collapse of a quiescent, nonmagnetized filamentary molecular cloud, taking into account the heating and cooling processes. At the initial state, the model cloud has the central density of nc=103 cm-3 and temperature of T=15 K. We follow its contraction until the central density reaches nc≃1010 cm-3. The cloud contracts mainly due to the CO line cooling and cooling by gas-dust interactions. During the contraction, the cloud temperature stays nearly constant at T~10 K because the net radiative cooling rate nearly balances with the heating by gravitational compression, which is the most efficient heating source. When the central density exceeds 104-105 cm-3, a shock wave is formed at r~0.05 pc. The shock wave separates the cloud into two parts, i.e., a dense spindle and a diffuse envelope. The spindle slowly contracts due to cooling by gas-dust interactions. During the contraction, the outer part of the spindle has a power-law density distribution of ρ∝r−2, which is different from that expected by an isothermal model,ρ∝r−4. Applying linear theory, we find that the collapsing spindle is likely to fragment into dense cores by the stage that the central density reaches nc~108-109 cm-3 if the amplitude of the perturbation is greater than A≳10−2. The masses of the dense cores depend on the initial amplitude of the perturbation. When the initial amplitude of the perturbation is small (A≲10−1), the spindle fragments into cores with mass 0.1-0.5 M☉. On the other hand, when the spindle has relatively large inhomogeneity (A~10−1), it fragments into cores with mass ~10 M☉.

Star Formation in the L1333 Molecular Cloud in Cassiopeia

?????Radio and optical observations have been made in order to examine star formation in the dark cloud L1333 [?(1950) = 2h210, ?(1950) = 75?15']. A study of the cumulative distribution of field star distance moduli yields a distance value close to 180 pc for the L1333 molecular cloud. 13CO observations revealed two filamentary molecular clouds with distinct velocities of VLSR ~ -2 and 3 km s-1, respectively. Thirteen C18O cores, characterized by an average molecular mass (MLTE) of ~9 M? and a mean density of ~1.4 ? 104 cm-3, are embedded in the two 13CO clouds. We have detected 18 H? emission-line stars projected within or near the 13CO clouds on objective-prism plates. They can be regarded as candidate pre?main-sequence stars formed in the clouds. Five IRAS sources with flux density distributions characteristic of young stellar objects (YSOs) are found in the area of the cloud. Three of the IRAS sources coincide with H? emission-line stars, the others with faint stars without detected H? emission. One of the latter sources is associated with a C18O core and exhibits a protostar-type infrared spectrum. The IRAS source exhibits a winglike feature of ~1.6 km s-1 (full width at zero intensity) in C18O, which may indicate that the YSO has an outflow. We note that the C18O core has the smallest ratio of virial mass (Mvir) to MLTE among the C18O cores in L1333, suggesting that star formation occurs preferentially in a core whose internal kinetic energy is low compared with the self-gravitational energy. A possible scenario for the past star formation activity is discussed.

Formation and Evolution of Filamentary Molecular Clouds with Oblique Magnetic Field

Formation and Evolution of Filamentary Molecular Clouds with Oblique Magnetic Field

Wave Motions in Molecular Clouds: Results in Two Dimensions

We study the linear evolution of small perturbations in self-gravitating fluid systems in two spatial dimensions; we consider both cylindrical and cartesian (i.e., slab) geometries. The treatment is general, but the application is to molecular clouds. We consider a class of equations of state which heuristically take into account the presence of turbulence; in particular, we consider equations of state which are {\it softer} than isothermal. We take the unperturbed cloud configuration to be in hydrostatic equilibrium. We find a class of wave solutions which propagate along a pressure supported cylinder (or slab) and have finite (trapped) spatial distributions in the direction perpendicular to the direction of propagation. Our results indicate that the dispersion relations for these two dimensional waves have similar forms for the two geometries considered here. Both cases possess a regime of instability and a fastest growing mode. We also find the (perpendicular) form of the perturbations for a wide range of wavelengths. Finally, we discuss the implications of our results for star formation and molecular clouds. The mass scales set by instabilities in both molecular cloud filaments and sheets are generally much larger than the masses of stars. However, these instabilities can determine the length scales for the initial conditions for protostellar collapse.

On the Global Stability of Magnetized Accretion Disks. II. Vertical and Azimuthal Magnetic Fields

We investigate the global stability of a differentially rotating fluid shell threaded by vertical and azimuthal magnetic fields to linear, axisymmetric perturbations. This system, which models a thick accretion disk in the vicinity of its midplane, is susceptible to the Velikhov-Chandrasekhar (VC) instability in the absence of the azimuthal field. In most cases, the azimuthal field tends to stabilize the VC instability, although strong fields (Alfv\'en speed of order the characteristic rotational speed in our incompressible model) are required for complete stabilization. Stability diagrams are constructed, indicating critical values of the two fields for instability. We find an additional strong field instability that arises when the azimuthal Alfv\'en speed exceeds the characteristic rotational speed. This instability, in the case of a freely bounded configuration, has certain similarities to the sausage instability for interpenetrating fields in plasma physics, and may be important for very massive disks or filamentary molecular clouds. An application to the L1641 region in Orion A is briefly discussed. Finally, we find that the effect of a radially varying vertical field (without an azimuthal field) is mainly stabilizing.

Fragmentation of filamentary molecular clouds with longitudinal magnetic fields: Formation of disks and their collapse

Fragmentation of filamentary molecular clouds with longitudinal magnetic fields: Formation of disks and their collapse

Effects of Magnetic Fields and Rotation on the Fragmentation of Filamentary Molecular Clouds: Comparison of the Theory with the Orion A Cloud: Erratum

Effects of Magnetic Fields and Rotation on the Fragmentation of Filamentary Molecular Clouds: Comparison of the Theory with the Orion A Cloud: Erratum

Molecular cloud cores in the Orion A cloud. I - Nobeyama CS (1-0) survey

A first high-resolution survey of molecular cloud cores in the Orion A giant molecular cloud is reported. We identified 125 molecular cloud cores from an analysis of the spatial and velocity distribution of the CS (1-0) emission. The cores are generally elongated along the filamentary molecular cloud, and the axial ratio is about 0.5. The mass spectrum index of the cores is -1.6 for M≥50 M ○ .. The physical properties of the cores identified in Orion are compared with those of cores in dark clouds reported in the literature. The average radius of the cores in the Orion A cloud, 0.16 pc, is comparable to that of the cores in dark clouds

Effects of magnetic fields and rotation on the fragmentation of filamentary molecular clouds - Comparison of the theory with the Orion A cloud

view Abstract Citations (54) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effects of Magnetic Fields and Rotation on the Fragmentation of Filamentary Molecular Clouds: Comparison of the Theory with the Orion A Cloud Hanawa, Tomoyuki ; Nakamura, Fumitaka ; Matsumoto, Tomoaki ; Nakano, Takenori ; Tatematsu, Ken'ichi ; Umemoto, Tomofumi ; Kameya, Osamu ; Hirano, Naomi ; Hasegawa, Tetsuo ; Kaifu, Norio ; Yamamoto, Satoshi Abstract We discuss the fragmentation of a filamentary molecular cloud on the basis of a magnetohydrodynamical stability analysis and the observations of the Orion A cloud with the Nobeyama 45 m telescope. Our model cloud has axial and helical magnetic fields and rotates around the axis. The dispersion relation for this cloud shows that the presence of a magnetic field and/or rotation shortens the wavelength of the most unstable mode, i.e., the mean distance between the adjacent fragments, measured in units of the filament diameter. We compare the theoretical results with the observed clumpy structure of the filamentary Orion A cloud and find that the sum of magnetic and centrifugal forces in the parent cloud was comparable with the pressure force (thermal and turbulent) and has had significant effects on the fragmentation. The rotation velocity of the filament measured in the (C-13)O emission line is consistent with this result. Publication: The Astrophysical Journal Pub Date: February 1993 DOI: 10.1086/186749 Bibcode: 1993ApJ...404L..83H Keywords: Fragmentation; H Ii Regions; Interstellar Magnetic Fields; Magnetohydrodynamic Stability; Molecular Clouds; Rotation; Carbon Monoxide; Emission Spectra; Line Spectra; Astrophysics; HYDROMAGNETICS; INSTABILITIES; ISM: INDIVIDUAL NAME: ORION NEBULA; ISM: MAGNETIC FIELDS full text sources ADS | data products SIMBAD (2) Related Materials (1) Erratum: 1993ApJ...412L..75H

A large molecular cloud toward the SNR W50 and SS 433

view Abstract Citations (34) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A large molecular cloud toward the SNR W50 and SS 433. Huang, Y. -L. ; Dame, T. M. ; Thaddeus, P. Abstract The CO 1 yields 0 transition at 115 GHz has been mapped over an area of more than 6 deg sq toward W50, the extended SNR surrounding the peculiar object SS 433. W50 is found to lie at the end of a filamentary molecular cloud 4 deg long and 1 deg wide that closely matches a conspicuous dust lane on the Palomar Sky Survey; the cloud's kinematic distance is within 0.7 of 2.2 kpc, in agreement with the distance 2-3.3 kpc estimated for the SNR, and its mass is about 120,000 solar masses. Though there is little evidence of star formation in the molecular filament, or evidence of an interaction between the filament and W50 (or SS 433), the positional coincidence of the two, their similar distance, and their displacement nearly two molecular scale heights from the plane all suggest a physical relationship or common origin. One possibility is that both the molecular filament and the stellar progenitor of W50 were ejected from a large active molecular complex lying in the plane at l = 40 deg. Publication: The Astrophysical Journal Pub Date: September 1983 DOI: 10.1086/161323 Bibcode: 1983ApJ...272..609H Keywords: Molecular Clouds; Nebulae; Radio Astronomy; Supernova Remnants; Carbon Monoxide; Sky Surveys (Astronomy); Stellar Evolution; Astrophysics full text sources ADS | data products SIMBAD (8)