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Mohanty S.,Imperial College London | Ercolano B.,Ludwig Maximilians University of Munich | Ercolano B.,Cluster of Excellence Origin and Structure of the Universe | Turner N.J.,Jet Propulsion Laboratory
Astrophysical Journal

We investigate the viability of the magnetorotational instability (MRI) in X-ray ionized viscous accretion disks around both solar-type stars and very low mass stars. In particular, we determine the disk regions where the MRI can be shut off either by Ohmic resistivity (the so-called dead and undead zones) or by ambipolar diffusion (a region we term the zombie zone). We consider two stellar masses: M * = 0.7 M ⊙ and 0.1 M ⊙. In each case, we assume that: the disk surface density profile is that of a scaled Minimum Mass Solar Nebula, with M disk/M * = 0.01 as suggested by current data; disk ionization is driven primarily by stellar X-rays, complemented by cosmic rays and radionuclides; and the stellar X-ray luminosity scales with bolometric luminosity as LX /L * ≈ 10-3.5, as observed. Ionization rates are calculated with the MOCCASIN Monte Carlo X-ray transport code, and ionization balance determined using a simplified chemical network, including well-mixed 0.1 μm grains at various levels of depletion. We find that (1) ambipolar diffusion is the primary factor controlling MRI activity in disks around both solar-type and very low mass classical T Tauri stars. Assuming that the MRI yields the maximum possible field strength at each radius, we further find that: (2) the MRI-active layer constitutes only ∼5%-10% of the total disk mass; (3) the accretion rate (M) varies radially in both magnitude and sign (inward or outward), implying time-variable accretion as well as the creation of disk gaps and overdensities, with consequences for planet formation and migration; (4) achieving the empirical accretion rates in solar-type and very low mass stars requires a depletion of well-mixed small grains (via grain growth and/or settling) by a factor of 10-1000 relative to the standard dust-to-gas mass ratio of 10-2; and (5) the current non-detection of polarized emission from field-aligned grains in the outer disk regions is consistent with active MRI at those radii. © 2013. The American Astronomical Society. All rights reserved. Source

Ercolano B.,University of Exeter | Ercolano B.,Universitats Sternwarte Munich | Ercolano B.,Cluster of Excellence Origin and Structure of the Universe | Gritschneder M.,Peking University
Monthly Notices of the Royal Astronomical Society

We compare the three-dimensional gas temperature distributions obtained by a dedicated radiative transfer and photoionization code, MOCASSIN, against those obtained by the recently developed smooth particle hydrodynamics (SPH) plus ionization code IVINE for snapshots of a hydrodynamical simulation of a turbulent interstellar medium (ISM) irradiated by a nearby O star. Our tests demonstrate that the global ionization properties of the region are correctly reproduced by IVINE, hence validating further application of this code to the study of feedback in star-forming regions. However, we highlight potentially important discrepancies in the detailed temperature distribution. In particular, we show that in the case of highly inhomogeneous density distributions, the commonly employed on-the-spot (OTS) approximation yields unrealistically sharp shadow regions which can affect the dynamical evolution of the system. We implement a simple strategy to include the effects of the diffuse field in future calculations, which makes use of physically motivated temperature calibrations of the diffusefield-dominated regions and can be readily applied to similar codes. We find that while the global qualitative behaviour of the system is captured by simulations with the OTS approximation, the inclusion of the diffuse field in IVINE (called DIVINE) results in a stronger confinement of the cold gas, leading to denser and less coherent structures. This in turn leads to earlier triggering of star formation.We confirm that turbulence is being driven in simulations that include the diffuse field, but the efficiency is slightly lower than in simulations that use the OTS approximation. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Baldovin-Saavedra C.,University of Geneva | Baldovin-Saavedra C.,Observatoire de Geneva | Audard M.,University of Geneva | Audard M.,Observatoire de Geneva | And 9 more authors.
Astronomy and Astrophysics

Context. The [Ne II] line 12.81 μm was proposed to be a good tracer of gas in the environments of proto-planetary disks; its origin is explained by different mechanisms: jets in outflows, photo-evaporative disk winds driven by stellar X-rays/EUV or by the X-ray irradiated proto-planetary disk atmosphere. Previous Spitzer studies gave hints toward the neon emitting mechanism by exploring correlations between the line luminosity and properties of the star-disk system. These studies concluded that the origin of the emission is likely related to accretion and outflows, with some influence from X-rays. Aims. We provide direct constraints on the origin of the [Ne II] emission using high-spatial and spectral resolution observations that allow us to study the kinematics of the emitting gas. In addition we compare the [Ne II] line with optical forbidden lines. Methods. We obtained high-resolution ground-based observations with VISIR-VLT for 15 stars and UVES-VLT for three of them. The stars were chosen for having bright neon emission lines detected with Spitzer/IRS. The velocity shifts and profiles are used to disentangle the different emitting mechanisms producing the [Ne II] line. A comparison between results from this study and previous high-resolution studies is also presented. Results. The [Ne II] line was detected in seven stars, among them the first confirmed detection of [Ne II] in a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate an origin in a jet. In two stars, the small shifts and asymmetric profiles indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to edge-on, shows a spatially unresolved line centered at the stellar rest velocity, although cross-dispersion centroids move within 10 AU from one side of the star to the other as a function of wavelength. The line profile is symmetric with wings extending up to ∼±80 km s -1. The origin of the [Ne II] line is unclear and could either be due to the bipolar jet or to the disk. For the stars with VLT-UVES observations, in several cases, the optical forbidden line profiles and shifts are very similar to the profile of the [Ne II] line, suggesting that the lines are emitted in the same region. A general trend observed with VISIR is a lower line flux when compared with the fluxes obtained with Spitzer. We found no correlation between the line full-width at half maximum and the line peak velocity. The [Ne II] line remains undetected in a large part of the sample, an indication that the emission detected with Spitzer in those stars is likely extended. © 2012 ESO. Source

Spezzi L.,European Space Agency | De Marchi G.,European Space Agency | Panagia N.,US Space Telescope Science Institute | Panagia N.,Supernova Ltd | And 4 more authors.
Monthly Notices of the Royal Astronomical Society

We present a multiwavelength study of three star-forming regions, spanning the age range 1-14Myr, located between the 30 Doradus complex and supernova SN 1987A in the Large Magellanic Cloud (LMC). We reliably identify about 1000 pre-main-sequence (PMS) star candidates actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (M). Our measurements represent the largest M data set of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical M of PMS stars in the LMC is higher than for galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and M estimates, the difference in M between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disc dispersal and find indications that typically higher M are to be expected in low-metallicity environments. However, many issues of this scenario need to be clarified by future observations and modelling. We also find that, in the mass range 1-2M ⊙, M of PMS stars in the LMC increases with stellar mass as M ∝ M * b, with b≈ 1, i.e. slower than the second power law found for galactic PMS stars in the same mass regime. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

Fabbri J.,University College London | Otsuka M.,US Space Telescope Science Institute | Barlow M.J.,University College London | Gallagher J.S.,Louisiana State University | And 9 more authors.
Monthly Notices of the Royal Astronomical Society

We present an analysis of multi-epoch observations of the Type II-P supernova SN 2004et. New and archival optical spectra of SN 2004et are used to study the evolution of the Hα and [Oi] 6300-Å line profiles between days 259 and 646. Mid-infrared imaging with Michelle on Gemini-North and with all three instruments of the Spitzer Space Telescope was carried out between 2004 and 2010, supplemented by archival Spitzer data. We include Spitzer'warm' mission photometry at 3.6 and 4.5μm obtained on days 1779, 1931 and 2151, along with ground-based and Hubble Space Telescope (HST) optical and near-infrared observations obtained between days 79 and 1803. Multiwavelength light curves are presented, as well as optical-infrared spectral energy distributions (SEDs) for multiple epochs. Starting from about day 300, the optical light curves provide evidence for an increasing amount of circumstellar extinction attributable to newly formed dust, with the additional extinction reaching 0.8-1.5mag in the V band by day 690. The overall SEDs were fitted with multiple blackbody components, in order to investigate the luminosity evolution of the supernova, and then with Monte Carlo radiative transfer models using smooth or clumpy dust distributions, in order to estimate how much new dust condensed in the ejecta. The luminosity evolution was consistent with the decay of 56Co in the ejecta up until about day 690, after which an additional emission source is required, in agreement with the findings of Kotak et al. Clumped dust density distributions consisting of 20percent amorphous carbons and 80percent silicates by mass were able to match the observed optical and infrared SEDs, with dust masses that increased from 8 × 10-5M⊙ on day 300 to 1.5 × 10-3M⊙ on day 690, still significantly lower than the values needed for core-collapse supernovae to make a significant contribution to the dust enrichment of galaxies. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

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