arXiv subjects

Tomoaki Matsumoto

Publications and source records attributed to Tomoaki Matsumoto.

At least 55 records · Page 3Linked to original sources

arXiv2017

Fates of the dense cores formed by fragmentation of filaments: do they fragment again or not?

Fragmentation of filaments into dense cores is thought to be an important step in forming stars. The bar-mode instability of spherically collapsing cores found in previous linear analysis invokes a possibility of re-fragmentation of the cores due to their ellipsoidal (prolate or oblate) deformation. To investigate this possibility, here we perform three-dimensional self-gravitational hydrodynamics simulations that follow all the way from filament fragmentation to subsequent core collapse. We assume the gas is polytropic with index γ, which determines the stability of the bar-mode. For the case that the fragmentation of isolated hydrostatic filaments is triggered by the most unstable fragmentation mode, we find the bar mode grows as collapse proceeds if γ< 1.1, in agreement with the linear analysis. However, it takes more than ten orders-of-magnitude increase in the central density for the distortion to become non-linear. In addition to this fiducial case, we also study non-fiducial ones such as the fragmentation is triggered by a fragmentation mode with a longer wavelength and it occurs during radial collapse of filaments and find the distortion rapidly grows. In most of astrophysical applications, the effective polytropic index of collapsing gas exceeds 1.1 before ten orders-of-magnitude increase in the central density. Thus, supposing the fiducial case of filament fragmentation, re-fragmentation of dense cores would not be likely and their final mass would be determined when the filaments fragment.

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arXiv2017

Circumstellar disks and outflows in turbulent molecular cloud cores: possible formation mechanism for misaligned systems

We investigate the formation of circumstellar disks and outflows subsequent to the collapse of molecular cloud cores with the magnetic field and turbulence. Numerical simulations are performed by using an adaptive mesh refinement to follow the evolution up to $\sim 1000$~yr after the formation of a protostar. In the simulations, circumstellar disks are formed around the protostars; those in magnetized models are considerably smaller than those in nonmagnetized models, but their size increases with time. The models with stronger magnetic field tends to produce smaller disks. During evolution in the magnetized models, the mass ratios of a disk to a protostar is approximately constant at $\sim 1-10$\%. The circumstellar disks are aligned according to their angular momentum, and the outflows accelerate along the magnetic field on the $10-100$~au scale; this produces a disk that is misaligned with the outflow. The outflows are classified into two types: a magneto-centrifugal wind and a spiral flow. In the latter, because of the geometry, the axis of rotation is misaligned with the magnetic field. The magnetic field has an internal structure in the cloud cores, which also causes misalignment between the outflows and the magnetic field on the scale of the cloud core. The distribution of the angular momentum vectors in a core also has a non-monotonic internal structure. This should create a time-dependent accretion of angular momenta onto the circumstellar disk. Therefore, the circumstellar disks are expected to change their orientation as well as their sizes in the long-term evolutions.

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arXiv2017

Spiral Arms, Infall, and Misalignment of the Circumbinary Disk from the Circumstellar Disks in the Protostellar Binary System L1551 NE

We report the ALMA Cycle 2 observations of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), 13CO (3-2), SO (7_8-6_7), and the CS (7-6) emission. At 0.18" (= 25 AU) resolution, ~4-times higher than that of our Cycle 0 observations, the circumbinary disk as seen in the 0.9-mm emission is shown to be comprised of a northern and a southern spiral arm, with the southern arm connecting to the circumstellar disk around Source B. The western parts of the spiral arms are brighter than the eastern parts, suggesting the presence of an m=1 spiral mode. In the C18O emission, the infall gas motions in the inter-arm regions and the outward gas motions in the arms are identified. These observed features are well reproduced with our numerical simulations, where gravitational torques from the binary system impart angular momenta to the spiral-arm regions and extract angular momenta from the inter-arm regions. Chemical differentiation of the circumbinary disk is seen in the four molecular species. Our Cycle 2 observations have also resolved the circumstellar disks around the individual protostars, and the beam-deconvolved sizes are 0.29" X 0.19" (= 40 X 26 AU) (P.A. = 144 deg) and 0.26" X 0.20" (= 36 X 27 AU) (P.A. = 147 deg) for Sources A and B, respectively. The position and inclination angles of these circumstellar disks are misaligned with that of the circumbinary disk. The C18O emission traces the Keplerian rotation of the misaligned disk around Source A.

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arXiv2016

Discovery of Infalling Motion with Rotation of the Cluster-Forming Clump S235AB and Its Implication to the Clump Structures

We report the discovery of infalling motion with rotation of S235AB the massive cluster-forming clump (~10^3 Mo) in the S235 region. Our C18O observations with the 45m telescope at the Nobeyama Radio Observatory have revealed the elliptical shape of the clump. Position-velocity (PV) diagram taken along its major axis exhibits two well-defined peaks symmetrically located with respect to the clump center, which is similar to that found for a dynamically infalling envelope with rotation around a single protostar modeled by N. Ohashi and his collaborators, indicating that the cluster-forming clump is also collapsing by the self-gravity toward the clump center. With analogue to Ohashi's model, we made a simple model of an infalling, rotating clump to fit the observed data. Based on the inferred model parameters as well as results of earlier observations and simulations in the literature, we discuss structures of the clump such as the relation among the global mass infall rate (~10^-3 Mo/yr), formation of a compact core (with a mass and size of ~4 Mo and < 0.1 pc) at the center, and a massive star (~11 Mo) forming in the core.

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arXiv2016

Formation of the Unequal-Mass Binary Protostars in L1551 NE by Rotationally-Driven Fragmentation

We present observations at 7 mm that fully resolve the two circumstellar disks, and a reanalyses of archival observations at 3.5 cm that resolve along their major axes the two ionized jets, of the class I binary protostellar system L1551 NE. We show that the two circumstellar disks are better fit by a shallow inner and steep outer power-law than a truncated power-law. The two disks have very different transition radii between their inner and outer regions of $\sim$18.6 AU and $\sim$8.9 AU respectively. Assuming that they are intrinsically circular and geometrically thin, we find that the two circumstellar disks are parallel with each other and orthogonal in projection to their respective ionized jets. Furthermore, the two disks are closely aligned if not parallel with their circumbinary disk. Over an interval of $\sim$10 yr, source B (possessing the circumsecondary disk) has moved northwards with respect to and likely away from source A, indicating an orbital motion in the same direction as the rotational motion of their circumbinary disk. All the aforementioned elements therefore share the same axis for their angular momentum, indicating that L1551 NE is a product of rotationally-driven fragmentation of its parental core. Assuming a circular orbit, the relative disk sizes are compatible with theoretical predictions for tidal truncation by a binary system having a mass ratio of $\sim$0.2, in agreement with the reported relative separations of the two protostars from the center of their circumbinary disk. The transition radii of both disks, however, are a factor of $\gtrsim$1.5 smaller than their predicted tidally-truncated radii.

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arXiv2016

Revealing a detailed mass distribution of a high-density core MC27/L1521F in Taurus with ALMA

We present the results of ALMA observations of dust continuum emission and molecular rotational lines toward a dense core MC27 (aka L1521F) in Taurus, which is considered to be at a very early stage of star formation. The detailed column density distribution on size scales from a few tens AU to ~10,000 AU scale are revealed by combining the ALMA (12 m array + 7 m array) data with the published/unpublished single-dish data. The high angular resolution observations at 0.87 mm with a synthesized beam size of ~0."74 x 0."32 reveal that a protostellar source, MMS-1, is not spatially resolved and lacks associated gas emission, while a starless high-density core, MMS-2, has substructures both in dust and molecular emission. The averaged radial column density distribution of the inner part of MC27/L1521F (r $\lesssim$ 3000 AU) is N(H2) ~r$^{-0.4}$, clearly flatter than that of the outer part, ~r$^{-1.0}$. The complex velocity/spatial structure obtained with previous ALMA observations is located inside the inner flatter region, which may reflect the dynamical status of the dense core.

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arXiv2016

Rotationally-Driven Fragmentation for the Formation of the Binary Protostellar System L1551 IRS 5

Either bulk rotation or local turbulence is widely invoked to drive fragmentation in collapsing cores so as to produce multiple star systems. Even when the two mechanisms predict different manners in which the stellar spins and orbits are aligned, subsequent internal or external interactions can drive multiple systems towards or away from alignment thus masking their formation process. Here, we demonstrate that the geometrical and dynamical relationship between the binary system and its surrounding bulk envelope provide the crucial distinction between fragmentation models. We find that the circumstellar disks of the binary protostellar system L1551 IRS 5 are closely parallel not just with each other but also with their surrounding flattened envelope. Measurements of the relative proper motion of the binary components spanning nearly 30 yr indicate an orbital motion in the same sense as the envelope rotation. Eliminating orbital solutions whereby the circumstellar disks would be tidally truncated to sizes smaller than are observed, the remaining solutions favor a circular or low-eccentricity orbit tilted by up to $\sim$25$^\circ$ from the circumstellar disks. Turbulence-driven fragmentation can generate local angular momentum to produce a coplanar binary system, but which bears no particular relationship with its surrounding envelope. Instead, the observed properties conform with predictions for rotationally-driven fragmentation. If the fragments were produced at different heights or on opposite sides of the midplane in the flattened central region of a rotating core, the resulting protostars would then exhibit circumstellar disks parallel with the surrounding envelope but tilted from the orbital plane as is observed.

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arXiv2015

An origin of arc structures deeply embedded in dense molecular cloud cores

We investigated the formation of arc-like structures in the infalling envelope around protostars, motivated by the recent Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the high-density molecular cloud core, MC27/L1521F. We performed self-gravitational hydrodynamical numerical simulations with an adaptive mesh refinement code. A filamentary cloud with a 0.1~pc width fragments into cloud cores because of perturbations due to weak turbulence. The cloud core undergoes gravitational collapse to form multiple protostars, and gravitational torque from the orbiting protostars produces arc structures extending up to a 1000~AU scale. As well as on a spatial extent, the velocity ranges of the arc structures, $\sim0.5\,\mathrm{km\,s}^{-1}$, are in agreement with the ALMA observations. We also found that circumstellar disks are often misaligned in triple system. The misalignment is caused by the tidal interaction between the protostars when they undergo close encounters because of a highly eccentric orbit of the tight binary pair.

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arXiv2015

Star formation in turbulent molecular clouds with colliding flow

Using self-gravitational hydrodynamical numerical simulations, we investigated the evolution of high-density turbulent molecular clouds swept by a colliding flow. The interaction of shock waves due to turbulence produces networks of thin filamentary clouds with a sub-parsec width. The colliding flow accumulates the filamentary clouds into a sheet cloud and promotes active star formation for initially high-density clouds. Clouds with a colliding flow exhibit a finer filamentary network than clouds without a colliding flow. The probability distribution functions (PDFs) for the density and column density can be fitted by lognormal functions for clouds without colliding flow. When the initial turbulence is weak, the column density PDF has a power-law wing at high column densities. The colliding flow considerably deforms the PDF, such that the PDF exhibits a double peak. The stellar mass distributions reproduced here are consistent with the classical initial mass function with a power-law index of $-1.35$ when the initial clouds have a high density. The distribution of stellar velocities agrees with the gas velocity distribution, which can be fitted by Gaussian functions for clouds without colliding flow. For clouds with colliding flow, the velocity dispersion of gas tends to be larger than the stellar velocity dispersion. The signatures of colliding flows and turbulence appear in channel maps reconstructed from the simulation data. Clouds without colliding flow exhibit a cloud-scale velocity shear due to the turbulence. In contrast, clouds with colliding flow show a prominent anti-correlated distribution of thin filaments between the different velocity channels, suggesting collisions between the filamentary clouds.

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arXiv2014

Colliding Filaments and a Massive Dense Core in the Cygnus OB 7 Molecular Cloud

We report results of molecular line observations carried out toward a massive dense core in the Cyg OB 7 molecular cloud. The core has an extraordinarily large mass ($\sim1.1 \times 10^4$ $M_\odot$) and size ($\sim2 \times 5$ pc$^2$), but there is no massive young star forming therein. We observed this core in various molecular lines such as C$^{18}$O($J=1-0$) using the 45m telescope at Nobeyama Radio Observatory. We find that the core has an elongated morphology consisting of several filaments and core-like structures. The filaments are massive ($10^2-10^3$ $M_\odot$), and they are apparently colliding against each other. Some candidates of YSOs are distributed around their intersection, suggesting that the collisions of the filaments may have influenced on their formation. To understand the formation and evolution of such colliding filaments, we performed numerical simulations using the adaptive mesh refinement (AMR) technique adopting the observed core parameters (e.g., the mass and size) as the initial conditions. Results indicate that the filaments are formed as seen in other earlier simulations for small cores in literature, but we could not reproduce the collisions of the filaments simply by assuming the large initial mass and size. We find that the collisions of the filaments occur only when there is a large velocity gradient in the initial core in a sense to compress it. We suggest that the observed core was actually compressed by an external effect, e.g., shocks of nearby supernova remnants including HB21 which has been suggested to be interacting with the Cyg OB 7 molecular cloud.

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arXiv2014

Angular Momentum Exchange by Gravitational Torques and Infall in the Circumbinary Disk of the Protostellar System L1551 NE

We report the ALMA observation of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), and 13CO (3-2) lines at a ~1.6 times higher resolution and a ~6 times higher sensitivity than those of our previous SMA observations, which revealed a r ~300 AU-scale circumbinary disk in Keplerian rotation. The 0.9-mm continuum shows two opposing U-shaped brightenings in the circumbinary disk, and exhibits a depression between the circumbinary disk and the circumstellar disk of the primary protostar. The molecular lines trace non-axisymmetric deviations from Keplerian rotation in the circumbinary disk at higher velocities relative to the systemic velocity, where our previous SMA observations could not detect the lines. In addition, we detect inward motion along the minor axis of the circumbinary disk. To explain the newly-observed features, we performed a numerical simulation of gas orbits in a Roche potential tailored to the inferred properties of L1551 NE. The observed U-shaped dust features coincide with locations where gravitational torques from the central binary system are predicted to impart angular momentum to the circumbinary disk, producing shocks and hence density enhancements seen as a pair of spiral arms. The observed inward gas motion coincides with locations where angular momentum is predicted to be lowered by the gravitational torques. The good agreement between our observation and model indicates that gravitational torques from the binary stars constitute the primary driver for exchanging angular momentum so as to permit infall through the circumbinary disk of L1551 NE.

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arXiv2014

ALMA observations of a high-density core in Taurus: dynamical gas interaction at the possible site of a multiple star formation

Starless dense cores eventually collapse dynamically, which forms protostars inside them, and the physical properties of the cores determine the nature of the forming protostars. We report ALMA observations of dust continuum emission and molecular rotational lines toward MC27 or L1521F, which is considered to be very close to the first protostellar core phase. We found a few starless high-density cores, one of which has a very high density of ~10^7 cm^-3, within a region of several hundred AU around a very low-luminosity protostar detected by Spitzer. A very compact bipolar outflow with a dynamical timescale of a few hundred years was found toward the protostar. The molecular line observation shows several cores with an arc-like structure, possibly due to the dynamical gas interaction. These complex structures revealed in the present observations suggest that the initial condition of star formation is highly dynamical in nature, which is considered to be a key factor in understanding fundamental issues of star formation such as the formation of multiple stars and the origin of the initial mass function of stars.

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arXiv2013

Conditions for Circumstellar Disk Formation: Effects of Initial Conditions and Sink Treatment

The formation of a circumstellar disk in collapsing cloud cores is investigated with three-dimensional magnetohydrodynamic simulations. We prepare four types of initial cloud having different density profiles and calculate their evolution with or without a sink. To investigate the effect of magnetic dissipation on disk formation, the Ohmic dissipation is considered in some models. Calculations show that disk formation is very sensitive to both the initial cloud configuration and the sink treatment. The disk size considerably differs in clouds with different density profiles even when the initial clouds have almost the same mass-to-flux ratio. Only a very small disk (\sim 10 AU in size) appears in clouds with a uniform density profile, whereas a large disk (\sim 100 AU in size) forms in clouds with a Bonnor-Ebert density profile. In addition, a large sink accretion radius numerically impedes disk formation during the main accretion phase and tends to foster the misleading notion that disk formation is completely suppressed by magnetic braking. The protostellar outflow is also greatly affected by the sink properties. A sink accretion radius of \lesssim 1 AU and sink threshold density of \gtrsim 10^ 13 cm^-3 are necessary for investigating disk formation during the main accretion phase.

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arXiv2012

Radiation Magnetohydrodynamic Simulations of Protostellar Collapse: Protostellar Core Formation

We report the first three-dimensional radiation magnetohydrodynamic (RMHD) simulations of protostellar collapse with and without Ohmic dissipation.We take into account many physical processes required to study star formation processes, including a realistic equation of state. We follow the evolution from molecular cloud cores until protostellar cores are formed with sufficiently high resolutions without introducing a sink particle. The physical processes involved in the simulations and adopted numerical methods are described in detail.We can calculate only about one year after the formation of the protostellar cores with our direct three-dimensional RMHD simulations because of the extremely short timescale in the deep interior of the formed protostellar cores, but successfully describe the early phase of star formation processes. The thermal evolution and the structure of the first and second (protostellar) cores are consistent with previous one-dimensional simulations using full radiation transfer, but differ considerably from preceding multi-dimensional studies with the barotropic approximation. The protostellar cores evolve virtually spherically symmetric in the ideal MHD models because of efficient angular momentum transport by magnetic fields, but Ohmic dissipation enables the formation of the circumstellar disks in the vicinity of the protostellar cores as in previous MHD studies with the barotropic approximation. The formed disks are still small (less than 0.35 AU) because we simulate only the earliest evolution. We also confirm that two different types of outflows are naturally launched by magnetic fields from the first cores and protostellar cores in the resistive MHD models.

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arXiv2012

Chemistry in the First Hydrostatic Core Stage By Adopting Three-Dimensional Radiation Hydrodynamic Simulations

We investigate molecular evolution from a molecular cloud core to a first hydrostatic core in three spatial dimensions. We perform a radiation hydrodynamic simulation in order to trace fluid parcels, in which molecular evolution is investigated, using a gas-phase and grain-surface chemical reaction network. We derive spatial distributions of molecular abundances and column densities in the core harboring the first core. We find that the total of gas and ice abundances of many species in a cold era (10 K) remain unaltered until the temperature reaches ~500 K. The gas abundances in the warm envelope and the outer layer of the first core (T < 500 K) are mainly determined via the sublimation of ice-mantle species. Above 500 K, the abundant molecules, such as H2CO, start to be destroyed, and simple molecules, such as CO, H2O and N2 are reformed. On the other hand, some molecules are effectively formed at high temperature; carbon-chains, such as C2H2 and cyanopolyynes, are formed at the temperature of >700 K. We also find that large organic molecules, such as CH3OH and HCOOCH3, are associated with the first core (r < 10 AU). Although the abundances of these molecules in the first core stage are comparable or less than in the protostellar stage (hot corino), reflecting the lower luminosity of the central object, their column densities in our model are comparable to the observed values toward the prototypical hot corino, IRAS 16293-2422. We propose that these large organic molecules can be good tracers of the first cores.

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arXiv2011

Impact of Protostellar Outflow on Star Formation: Effects of Initial Cloud Mass

Star formation efficiency controlled by the protostellar outflow in a single cloud core is investigated by three-dimensional resistive MHD simulations. Starting from the prestellar cloud core, the star formation process is calculated until the end of the main accretion phase. In the calculations, the mass of the prestellar cloud is parameterized. During the star formation, the protostellar outflow is driven by the circumstellar disk. The outflow extends also in the transverse direction until its width becomes comparable to the initial cloud scale, and thus, the outflow has a wide opening angle of >40 degrees. As a result, the protostellar outflow sweeps up a large fraction of the infalling material and ejects it into the interstellar space. The outflow can eject at most over half of the host cloud mass, significantly decreasing star formation efficiency. The outflow power is stronger in clouds with a greater initial mass. Thus, the protostellar outflow effectively suppresses star formation efficiency in a massive cloud. The outflow weakens significantly and disappears in several free-fall timescales of the initial cloud after the cloud begins to collapse. The natal prestellar core influences the lifetime and size of the outflow. At the end of the main accretion phase, a massive circumstellar disk comparable in mass to the protostar remains. Calculations show that typically, ~30% of the initial cloud mass is converted into the protostar and ~20% remains in the circumstellar disk, while ~40% is ejected into the interstellar space by the protostellar outflow. Therefore, a single cloud core typically has a star formation efficiency of 30-50%.

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arXiv2011

Effect of Magnetic Braking on the Circumstellar Disk Formation in a Strongly Magnetized Cloud

Using resistive magnetohydrodynamics simulation, we investigate circumstellar disk formation in a strongly magnetized cloud. As the initial state, an isolated cloud core embedded in a low-density interstellar medium with a uniform magnetic field is adopted. The cloud evolution is calculated until almost all gas inside the initial cloud falls onto either the circumstellar disk or a protostar, and a part of the gas is ejected into the interstellar medium by the protostellar outflow driven by the circumstellar disk. In the early main accretion phase, the disk size is limited to \sim 10 AU because the angular momentum of the circumstellar disk is effectively transferred by both magnetic braking and the protostellar outflow. In the later main accretion phase, however, the circumstellar disk grows rapidly and exceeds 100 AU by the end of the main accretion phase. This rapid growth of the circumstellar disk is caused by the depletion of the infalling envelope, while magnetic braking is effective when the infalling envelope is more massive than the circumstellar disk. The infalling envelope cannot brake the circumstellar disk when the latter is more massive than the former. In addition, the protostellar outflow weakens and disappears in the later main accretion phase, because the outflow is powered by gas accretion onto the circumstellar disk. Although the circumstellar disk formed in a magnetized cloud is considerably smaller than that in an unmagnetized cloud, a circumstellar disk exceeding 100 AU can form even in a strongly magnetized cloud.

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arXiv2011

An Implicit Scheme for Ohmic Dissipation with Adaptive Mesh Refinement

An implicit method for the ohmic dissipation is proposed. The proposed method is based on the Crank-Nicolson method and exhibits second-order accuracy in time and space. The proposed method has been implemented in the SFUMATO adaptive mesh refinement (AMR) code. The multigrid method on the grids of the AMR hierarchy converges the solution. The convergence is fast but depends on the time step, resolution, and resistivity. Test problems demonstrated that decent solutions are obtained even at the interface between fine and coarse grids. Moreover, the solution obtained by the proposed method shows good agreement with that obtained by the explicit method, which required many time steps. The present method reduces the number of time steps, and hence the computational costs, as compared with the explicit method.

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