arXiv subjects

Susana Lizano

Publications and source records attributed to Susana Lizano.

At least 37 records · Page 2Linked to original sources

arXiv2014

Gravitational Collapse and Disk Formation in Magnetized Cores

We discuss the effects of the magnetic field observed in molecular clouds on the process of star formation, concentrating on the phase of gravitational collapse of low-mass dense cores, cradles of sunlike stars. We summarize recent analytic work and numerical simulations showing that a substantial level of magnetic field diffusion at high densities has to occur in order to form rotationally supported disks. Furthermore, newly formed accretion disks are threaded by the magnetic field dragged from the parent core during the gravitational collapse. These disks are expected to rotate with a sub-Keplerian speed because they are partially supported by magnetic tension against the gravity of the central star. We discuss how sub-Keplerian rotation makes it difficult to eject disk winds and accelerates the process of planet migration. Moreover, magnetic fields modify the Toomre criterion for gravitational instability via two opposing effects: magnetic tension and pressure increase the disk local stability, but sub-Keplerian rotation makes the disk more unstable. In general, magnetized disks are more stable than their nonmagnetic counterparts; thus, they can be more massive and less prone to the formation of giant planets by gravitational instability.

astro-ph.GA↗

arXiv2014

Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet

We present $^{12}$CO(2-1) line and 1300 $μ$m continuum observations made with the Submillimeter Array (SMA) of the young star DG Tau B. We find, in the continuum observations, emission arising from the circumstellar disk surrounding DG Tau B. The $^{12}$CO(2-1) line observations, on the other hand, revealed emission associated with the disk and the asymmetric outflow related with this source. Velocity asymmetries about the flow axis are found over the entire length of the flow. The amplitude of the velocity differences is of the order of 1 -- 2 km s$^{-1}$ over distances of about 300 -- 400 AU. We interpret them as a result of outflow rotation. The sense of the outflow and disk rotation is the same. Infalling gas from a rotating molecular core cannot explain the observed velocity gradient within the flow. Magneto-centrifugal disk winds or photoevaporated disk winds can produce the observed rotational speeds if they are ejected from a keplerian disk at radii of several tens of AU. Nevertheless, these slow winds ejected from large radii are not very massive, and cannot account for the observed linear momentum and angular momentum rates of the molecular flow. Thus, the observed flow is probably entrained material from the parent cloud. DG Tau B is a good laboratory to model in detail the entrainment process and see if it can account for the observed angular momentum.

astro-ph.SR↗

arXiv2013

The compact, time-variable radio source projected inside W3(OH): Evidence for a Photoevaporated Disk?

We present new Karl G. Jansky Very Large Array observations of the compact (~ 0.05"), time-variable radio source projected near the center of the ultracompact HII region W3(OH). The analysis of our new data as well as of VLA archival observations confirms the variability of the source on timescales of years and for a given epoch indicates a spectral index of α= 1.3 +- 0.3 (S_ν\propto ν^α). This spectral index and the brightness temperature of the source (~6,500 K) suggest that we are most probably detecting partially optically thick free-free radiation. The radio source is probably associated with the ionizing star of W3(OH), but an interpretation in terms of an ionized stellar wind fails because the detected flux densities are orders of magnitude larger than expected. We discuss several scenarios and tentatively propose that the radio emission could arise in a static ionized atmosphere around a fossil photoevaporated disk.

astro-ph.SR↗

arXiv2013

Deciphering the ionized gas content in the massive star forming complex G75.78+0.34

We present sub-arcsecond observations toward the massive star forming region G75.78+0.34. We used the Very Large Array to study the centimeter continuum and H2O and CH3OH maser emission, and the Owens Valley Radio Observatory and Submillimeter Array to study the millimeter continuum and recombination lines (H40alpha and H30alpha). We found radio continuum emission at all wavelengths, coming from three components: (1) a cometary ultracompact (UC) HII region with an electron density 3.7x10^4 cm^{-3}, excited by a B0 type star, and with no associated dust emission; (2) an almost unresolved UCHII region (EAST), located 6 arcsec to the east of the cometary UCHII region, with an electron density 1.3x10^5 cm^{-3}, and associated with a compact dust clump detected at millimeter and mid-infrared wavelengths; and (3) a compact source (CORE), located 2 arcsec to the southwest of the cometary arc, with a flux density increasing with frequency, and embedded in a dust condensation of 30 Msun. The CORE source is resolved into two compact and unresolved sources which can be well-fit by two homogeneous hypercompact HII regions each one photo-ionized by a B0.5 ZAMS star, or by free-free radiation from shock-ionized gas resulting from the interaction of a jet/outflow system with the surrounding environment. The spatial distribution and kinematics of water masers close to the CORE-N and S sources, together with excess emission at 4.5 mum and the detected dust emission, suggest that the CORE source is a massive protostar driving a jet/outflow.

astro-ph.GA↗

arXiv2011

Magnetic Interactions in Pre-Main-Sequence Binaries

Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy $E_{\rm int}$ as a function of the stellar radii, the stellar magnetic field strengths, the binary semi-major axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variability in time and energetics of the synchrotron radiation depend on the details of the annihilation of magnetic fields through reconnection events, which generate electric fields that accelerate charged particles, and how those charged particles, especially fast electrons, are removed from the interaction region. However, the major qualitative features are well described by the background changes in the global magnetic configuration driven by the orbital motion. The theory can be tested by observing a collection of pre-main-sequence binary systems.

astro-ph.SR↗

arXiv2010

Stability of Magnetized Disks and Implications for Planet Formation

This paper considers gravitational perturbations in geometrically thin disks with rotation curves dominated by a central object, but with substantial contributions from magnetic pressure and tension. The treatment is general, but the application is to the circumstellar disks that arise during the gravitational collapse phase of star formation. We find the dispersion relation for spiral density waves in these generalized disks and derive the stability criterion for axisymmetric $(m=0)$ disturbances (the analog of the Toomre parameter $Q_T$) for any radial distribution of the mass-to-flux ratio $λ$. The magnetic effects work in two opposing directions: on one hand, magnetic tension and pressure stabilize the disk against gravitational collapse and fragmentation; on the other hand, they also lower the rotation rate making the disk more unstable. For disks around young stars the first effect generally dominates, so that magnetic fields allow disks to be stable for higher surface densities and larger total masses. These results indicate that magnetic fields act to suppress the formation of giant planets through gravitational instability. Finally, even if gravitational instability can form a secondary body, it must lose an enormous amount of magnetic flux in order to become a planet; this latter requirement represents an additional constraint for planet formation via gravitational instability and places a lower limit on the electrical resistivity.

astro-ph.GA↗

arXiv2010

An Expanding HI Photodissociated Region Associated with the Compact HII Region G213.880-11.837 in the GGD 14 Complex

We present high angular and spectral resolution HI 21~cm line observations toward the cometary-shaped compact HII region G213.880-11.837 in the GGD~14 complex.The kinematics and morphology of the photodissociated region, traced by the HI line emission, reveal that the neutral gas is part of an expanding flow. The kinematics of the HI gas along the major axis of G213.880-11.837 shows that the emission is very extended toward the SE direction, reaching LSR radial velocities in the tail of about 14 km/s. The ambient LSR radial velocity of the molecular gas is 11.5 km/s, which suggests a champagne flow of the HI gas. This is the second (after G111.61+0.37) cometary HII/HI region known.

astro-ph.GA↗

arXiv2009

Migration of Extrasolar Planets: Effects from X-Wind Accretion Disks

Magnetic fields are dragged in from the interstellar medium during the gravitational collapse that forms star/disk systems. Consideration of mean field magnetohydrodynamics (MHD) in these disks shows that magnetic effects produce subkeplerian rotation curves and truncate the inner disk. This letter explores the ramifications of these predicted disk properties for the migration of extrasolar planets. Subkeplerian flow in gaseous disks drives a new migration mechanism for embedded planets and modifies the gap opening processes for larger planets. This subkeplerian migration mechanism dominates over Type I migration for sufficiently small planets (m_P < 1 M_\earth) and/or close orbits (r < 1 AU). Although the inclusion of subkeplerian torques shortens the total migration time by only a moderate amount, the mass accreted by migrating planetary cores is significantly reduced. Truncation of the inner disk edge (for typical system parameters) naturally explains final planetary orbits with periods P=4 days. Planets with shorter periods P=2 days can be explained by migration during FU-Ori outbursts, when the mass accretion rate is high and the disk edge moves inward. Finally, the midplane density is greatly increased at the inner truncation point of the disk (the X-point); this enhancement, in conjunction with continuing flow of gas and solids through the region, supports the in situ formation of giant planets.

astro-ph.EP↗

arXiv2008

Collapsing Hot Molecular Cores: A Model for the Dust Spectrum and Ammonia Line Emission of the G31.41+0.31 Hot Core

We present a model aimed to reproduce the observed spectral energy distribution (SED) as well as the ammonia line emission of the G31.41+0.31 hot core. The core is modeled as an infalling envelope onto a massive star that is undergoing an intense accretion phase. We assume an envelope with a density and velocity structure resulting from the dynamical collapse of a singular logatropic sphere. The stellar and envelope physical properties are determined by fitting the SED. From these physical conditions, the ammonia line emission is calculated and compared with subarcsecond resolution VLA data of the (4,4) transition. The only free parameter in this line fitting is the ammonia abundance. The observed properties of the NH3(4,4) lines and their spatial distribution can be well reproduced provided it is taken into account the steep increase of the abundance in the hotter (> 100 K), inner regions of the core produced by the sublimation of icy mantles where ammonia molecules are trapped. The model predictions for the (2,2), (4,4), and (5,5) transitions are also in reasonably agreement with the single-dish spectra available in the literature. The best fit is obtained for a model with a star of 25 Msun, a mass accretion rate of 0.003 Msun/yr, and a total luminosity of 200,000 Lsun. The gas-phase ammonia abundance ranges from 2 \times 10^{-8} in the outer region to 3 \times 10^{-6} in the inner region. To our knowledge, this is the first time that the dust and molecular line data of a hot molecular core, including subarcsecond resolution data that spatially resolve the structure of the core, have been simultaneously explained by a physically self-consistent model. This modeling shows that massive protostars are able to excite high excitation ammonia transitions up to the outer edge (30,000 AU) of the large scale envelope.

astro-ph↗

arXiv2008

The Challenge of Sub-Keplerian Rotation for Disk Winds

Strong magnetization makes the disks surrounding young stellar objects rotate at rates that are too sub-Keplerian to enable the thermal launching of disk winds from their surfaces unless the rate of gas diffusion across field lines is dynamically fast. This under-appreciated implication of disk magnetization poses a considerable challenge for disk-wind theory.

astro-ph↗

arXiv2008

Monitoring the Large Proper Motions of Radio Sources in the Orion BN/KL Region

We present absolute astrometry of four radio sources in the Becklin-Neugebauer/Kleinman-Low (BN/KL) region, derived from archival data (taken in 1991, 1995, and 2000) as well as from new observations (taken in 2006). All data consist of 3.6 cm continuum emission and were taken with the Very Large Array in its highest angular resolution A configuration. We confirm the large proper motions of the BN object, the radio source I (GMR I) and the radio counterpart of the infrared source n (Orion-n), with values from 15 to 26 km/s. The three sources are receding from a point between them from where they seem to have been ejected about 500 years ago, probably via the disintegration of a multiple stellar system. We present simulations of very compact stellar groups that provide a plausible dynamical scenario for the observations. The radio source Orion-n appeared as a double in the first three epochs, but as single in 2006. We discuss this morphological change. The fourth source in the region, GMR D, shows no statistically significant proper motions. We also present new, accurate relative astrometry between BN and radio source I that restrict possible dynamical scenarios for the region. During the 2006 observations, the radio source GMR A, located about 1' to the NW of the BN/KL region, exhibited an increase in its flux density of a factor of ~3.5 over a timescale of one hour. This rapid variability at cm wavelengths is similar to that previously found during a flare at millimeter wavelengths that took place in 2003.

astro-ph↗

arXiv2007

Formation of OB Associations in Galaxies

We consider the formation of OB associations from two perspectives: (a) the fractional gas consumption in star formation,epsilon, per dynamical time scale t_dyn in a galaxy, and (b) the origin of the so-called Kennicutt-Schmidt law that the rate of star formation per unit area is proportional to a power, alpha, of the surface density in HI and H_2 gas when certain thresholds are crossed. The empirical findings that epsilon is approximately 0.01 and alpha is approximately 1.4 or 1.5 have simple explanations if the rate of star formation is magnetically regulated. An empirical test of the ideas resides in an analysis of why giant OB associations are ``strung out like pearls along the arms" of spiral galaxies.

astro-ph↗

arXiv2007

Mean-Field Magnetohydrodynamics of Accretion Disks

We consider the accretion process in a disk with magnetic fields that are dragged in from the interstellar medium by gravitational collapse. Two diffusive processes are at work in the system: (1) "viscous" torques exerted by turbulent and magnetic stresses, and (2) "resistive" redistribution of mass with respect to the magnetic flux arising from the imperfect conduction of current. In steady state, self-consistency between the two rates of drift requires that a relationship exists between the coefficients of turbulent viscosity and turbulent resistivity. Ignoring any interactions with a stellar magnetosphere, we solve the steady-state equations for a magnetized disk under the gravitational attraction of a mass point and threaded by an amount of magnetic flux consistent with calculations of magnetized gravitational collapse in star formation. Our model mean-field equations have an exact analytical solution that corresponds to magnetically diluted Keplerian rotation about the central mass point. The solution yields the strength of the magnetic field and the surface density as functions of radial position in the disk and their connection with the departure from pure Keplerian rotation in representative cases. We compare the predictions of the theory with the available observations concerning T Tauri stars, FU Orionis stars, and low- and high-mass protostars. Finally, we speculate on the physical causes for high and low states of the accretion disks that surround young stellar objects. One of the more important results of this study is the physical derivation of analytic expressions for the turbulent viscosity and turbulent resistivity.

astro-ph↗

arXiv2005

Dynamical decay of a massive multiple system in Orion KL?

We present absolute astrometry of 35 radio sources in the Orion Trapezium and Becklin-Neugebauer/Kleinman-Low regions, obtained from Very Large Array archival observations collected over a period of 15 years. By averaging the results for all the sources, we estimate the mean absolute proper motion of Orion to be --in Galactic coordinates-- $μ_\ell \cos b$ = +2.1 $\pm$ 0.2 mas yr$^{-1}$; $μ_b$ = $-$0.1 $\pm$ 0.2 mas yr$^{-1}$. These values agree remarkably well with those expected from the differential rotation of the Milky Way. Subtraction of this mean motion from the individual measurements allows us to register all proper motions to the rest frame of the Orion nebula, and identify radio sources with large residual velocities. In the KL region, we find three sources in this situation: the BN object, the radio source I, and the radio counterpart of the infrared source n. All three objects appear to be moving away from a common point where they must all have been located about 500 years ago. This suggests that all three sources were originally part of a multiple massive stellar system that recently disintegrated as a result of a close dynamical

astro-ph↗

arXiv2005

Proper Motions of the BN Object and the I Radio Source in Orion: Where and When Did BN Become a Runaway Star?

We present absolute astrometry of the core of the Orion molecular cloud, made with Very Large Array archive data taken over the last two decades. Our analysis reveals that both the BN object and the radio source I have proper motions: the BN object has a proper motion of $12.6 \pm 0.6$ mas yr$^{-1}$ (corresponding to a velocity of $27 \pm 1$ km s$^{-1}$ at an adopted distance of 450 pc) to the northwest, while the radio source I has a proper motion of $5.6 \pm 0.7$ mas yr$^{-1}$ (corresponding to a velocity of $12 \pm 2$ km s$^{-1}$) to the southeast. The motion of the two sources is nearly antiparallel, diverging from a point in between them, where they were located about 500 years ago. These results suggest that the BN object and the radio source I were part of a multiple young stellar system that disintegrated in the recent past.

astro-ph↗

arXiv2000

Singular Isothermal Disks: II. Nonaxisymmetric Bifurcations and Equilibria

We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance from the rotation axis of the system. In agreement with earlier analysis by Syer and Tremaine, we find that lopsided (M=1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into M=2, 3, 4, ... separate bodies. On the basis of our experience in this paper, and the preceding Paper I, we advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse.

astro-ph↗

arXiv1999

Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust

We construct detailed vertical structure models of irradiated accretion disks around T Tauri stars with interstellar medium dust uniformly mixed with gas. The dependence of the structure and emission properties on mass accretion rate, viscosity parameter, and disk radius is explored using these models. The theoretical spectral energy distributions (SEDs) and images for all inclinations are compared with observations of the entire population of Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we find that the median near-infrared fluxes can be explained within the errors with the most recent values for the median accretion rates for CTTS. We further show that the majority of the Class I sources in Taurus cannot be Class II sources viewed edge-on because they are too luminous and their colors would be consistent with disks seen only in a narrow range of inclinations. Our models appear to be too geometrically thick at large radii, as suggested by: (a) larger far-infrared disk emission than in the typical SEDs of T Tauri stars; (b) wider dark dust lanes in the model images than in the images of HH30 and HK Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than consistent with current surveys. The large thickness of the model is a consequence of the assumption that dust and gas are well-mixed, suggesting that some degree of dust settling may be required to explain the observations.

astro-ph↗

arXiv1999

Massive Stars: Their Environment and Formation

Cloud environment is thought to play a critical role in determining the mechanism of formation of massive stars. In this contribution we review the physical characteristics of the environment around recently formed massive stars. Particular emphasis is given to recent high angular resolution observations which have improved our knowledge of the physical conditions and kinematics of compact regions of ionized gas and of dense and hot molecular cores associated with luminous O and B stars. We will show that this large body of data, gathered during the last decade, has allowed significant progress in the understanding of the physical processes that take place during the formation and early evolution of massive stars.

astro-ph↗