Browsing by Subject "disks"

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Circumbinary Planet Formation In The Kepler-16 System. II. A Toy Model For In Situ Planet Formation Within A Debris Belt
(2014-07) Meschiari, Stefano; Meschiari, Stefano
Recent simulations have shown that the formation of planets in circumbinary configurations (such as those recently discovered by Kepler) is dramatically hindered at the planetesimal accretion stage. The combined action of the binary and the protoplanetary disk acts to raise impact velocities between kilometer-sized planetesimals beyond their destruction threshold, halting planet formation within at least 10 AU from the binary. It has been proposed that a primordial population of "large" planetesimals (100 km or more in size), as produced by turbulent concentration mechanisms, would be able to bypass this bottleneck; however, it is not clear whether these processes are viable in the highly perturbed circumbinary environments. We perform two-dimensional hydrodynamical and N-body simulations to show that kilometer-sized planetesimals and collisional debris can drift and be trapped in a belt close to the central binary. Within this belt, planetesimals could initially grow by accreting debris, ultimately becoming "indestructible" seeds that can accrete other planetesimals in situ despite the large impact speeds. We find that large, indestructible planetesimals can be formed close to the central binary within 10(5) yr, therefore showing that even a primordial population of "small" planetesimals can feasibly form a planet.
CO Rovibrational Emission As A Probe Of Inner Disk Structure
(2011-12) Salyk, Colette; Blake, Geoffrey A.; Boogert, Adwin C. A.; Brown, Joanna M.; Salyk, Colette
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width similar to 0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust.
Color Variability Of HBC 722 In The Post-Outburst Phases
(2015) Baek, Giseon; Pak, Soojong; Green, Joel D.; Meschiari, Stefano; Lee, Jeong-Eun; Jeon, Yiseul; Choi, Changsu; Im, Myungshin; Sung, Hyun-Il; Park, Won-Kee; Green, Joel D.; Meschiari, Stefano
We carried out photometric observations for HBC 722 in the Sloan Digital Sky Survey r, i, and z bands from 2011 April to 2013 May with the Camera for QUasars in EArly uNiverse attached to the 2.1 m Otto Struve telescope at McDonald Observatory. The post-outburst phenomena were classified into five phases according to not only brightness but also color variations, which might be caused by physical changes in the emitting regions of optical and near-infrared bands. A series of spectral energy distributions (SEDs) is presented to support color variations and track the time evolution of the SED in optical/near-infrared bands after the outburst. Given two years of data, possible periodicities of r, i, and z bands were checked. We found three families of signals around similar to 6, similar to 10, and similar to 1 days in three bands, which is broadly consistent with Green et al. We also examined short-term variability (intra-day and day scales) to search for evidences of flickering by using the micro-variability method. We found clear signs of day scale variability and weak indications of intra-day scale fluctuations, which implies that the flickering event occurs in HBC 722 after outburst.
DIGIT Survey Of Far-Infrared Lines From Protoplanetary Disks I. Oi , Cii , Oh, H2O, And CH+
(2013-11) Fedele, Davide; Bruderer, Simon; van Dishoeck, Ewine F.; Carr, J.; Herczeg, G. J.; Salyk, C.; Evans, Neal J.; Bouwman, J.; Meeus, Gwendolyn; Henning, Thomas; Green, Joel; Najita, J. R.; Gudel, M.; Evans, Neal J.; Green, Joel
We present far-infrared (50-200 mu m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [OI], [CII], CO, OH, H2O, CH+. The most common feature is the [OI] 63 mu m line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10(13) < N-OH < 10(16) cm(-2), emitting radii 15 < r < 100 AU and excitation temperatures 100 < T-ex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n greater than or similar to 10(10) cm(-3)) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near-to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T-ex similar to 100 K) and compact (r similar to 60 AU) emission in the case of HD 100546. Off-source [OI] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([OI] 63 mu m, [OI] 145 mu m, [CII]) are analyzed with PDR models and require high gas density (n greater than or similar to 10(5) cm(-3)) and high UV fluxes (G(o) similar to 10(3)-10(7)), consistent with a disk origin for the oxygen lines for most of the sources.
DIGIT Survey Of Far-Infrared Lines From Protoplanetary Disks I. Oi , Cii , Oh, H2O, And CH+
(2013-11) Fedele, Davide; Bruderer, Simon; van Dishoeck, Ewine F.; Carr, J.; Herczeg, G. J.; Salyk, C.; Evans, Neal J.; Bouwman, J.; Meeus, Gwendolyn; Henning, Thomas; Green, Joel; Najita, J. R.; Gudel, M.; Evans, Neal J.; Green, Joel
We present far-infrared (50-200 mu m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [OI], [CII], CO, OH, H2O, CH+. The most common feature is the [OI] 63 mu m line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10(13) < N-OH < 10(16) cm(-2), emitting radii 15 < r < 100 AU and excitation temperatures 100 < T-ex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n greater than or similar to 10(10) cm(-3)) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near-to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T-ex similar to 100 K) and compact (r similar to 60 AU) emission in the case of HD 100546. Off-source [OI] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([OI] 63 mu m, [OI] 145 mu m, [CII]) are analyzed with PDR models and require high gas density (n greater than or similar to 10(5) cm(-3)) and high UV fluxes (G(o) similar to 10(3)-10(7)), consistent with a disk origin for the oxygen lines for most of the sources.
Disentangling the Environment of the FU Orionis Candidate HBC 722 with Herschel
(2011-04) Green, Joel D.; Evans, Neal J., II; Kospal, Agnes; van Kempen, Tim A.; Herczeg, Gregory; Quanz, Sascha P.; Henning, Thomas; Lee, Jeong-Eun; Dunham, Michael M.; Meeus, Gwendolyn; Bouwman, Jeroen; van Dishoeck, Ewine; Chen, Jo-Hsin; Guedel, Manuel; Skinner, Stephen L.; Merello, Manuel; Pooley, David; Rebull, Luisa M.; Guieu, Sylvain; Green, Joel D.; Evans, Neal J., II
We analyze the submillimeter emission surrounding the new FU Orionis-type object, HBC 722. We present the first epoch of observations of the active environs of HBC 722, with imaging and spectroscopy from PACS, SPIRE, and HIFI on board the Herschel Space Observatory, as well as CO J = 2-1 and 350 mu m imaging (SHARC-II) with the Caltech Submillimeter Observatory. The primary source of submillimeter continuum emission in the region-2MASS 20581767+4353310-is located 16 '' south-southeast of the optical flaring source while the optical and near-infrared emission is dominated by HBC 722. A bipolar outflow extends over HBC 722; the most likely driver is the submillimeter source. We detect warm (100 K) and hot (246 K) CO emission in the surrounding region, evidence of outflow-driven heating in the vicinity. The region around HBC 722 itself shows little evidence of heating driven by the new outbursting source itself.
The Dual Origin Of The Nitrogen Deficiency In Comets: Selective Volatile Trapping In The Nebula And Postaccretion Radiogenic Heating
(2012-10) Mousis, Olivier; Guilbert-Lepoutre, Aurelie; Lunine, Jonathan I.; Cochran, Anita L.; Waite, J. Hunter; Petit, Jean-Marc; Rousselot, Philippe; Cochran, Anita L.
We propose a scenario that explains the apparent nitrogen deficiency in comets in away that is consistent with the fact that the surfaces of Pluto and Triton are dominated by nitrogen-rich ice. We use a statistical thermodynamic model to investigate the composition of the successive multiple guest clathrates that may have formed during the cooling of the primordial nebula from the most abundant volatiles present in the gas phase. These clathrates agglomerated with the other ices (pure condensates or stoichiometric hydrates) and formed the building blocks of comets. We report that molecular nitrogen is a poor clathrate former, when we consider a plausible gas-phase composition of the primordial nebula. This implies that its trapping into cometesimals requires a low disk temperature (similar to 20 K) in order to allow the formation of its pure condensate. We find that it is possible to explain the lack of molecular nitrogen in comets as a consequence of their postformation internal heating engendered by the decay of short-lived radiogenic nuclides. This scenario is found to be consistent with the presence of nitrogen-rich ice covers on Pluto and Triton. Our model predicts that comets should present xenon-to-water and krypton-to-water ratios close to solar xenon-to-oxygen and krypton-to-oxygen ratios, respectively. In contrast, the argon-to-water ratio is predicted to be depleted by a factor of similar to 300 in comets compared to solar argon-to-oxygen, as a consequence of poor trapping efficiency and radiogenic heating.
Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association
(2015-01) Liu, Yao; Herczeg, Gregory J.; Gong, Munan A.; Allers, Katelyn N.; Brown, Joanna M.; Kraus, Adam L.; Liu, Michael C.; Shkolnik, Evgenya L.; van Dishoeck, Ewine F.; Kraus, Adam L.
We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.
High-Resolution Far-Infrared Studies Of Intermediate-Mass Pre-Main-Sequence Objects
(1998-12) Di Francesco, James; Evans, Neal J.; Harvey, Paul M.; Mundy, Lee G.; Butner, Harold M.; Di Francesco, James; Evans, Neal J.; Harvey, Paul M.
We have obtained high-resolution far-infrared maps of nine regions with 10 Herbig Ae/Be stars (intermediate-mass pre-main-sequence stars). Similar maps were obtained for 10 embedded IRAS sources with S-v(100 mu m) > S-v(60 mu m) and L similar to 200 L., which are possible evolutionary precursors of Herbig Ae/Be stars. Single far-infrared sources were found in most maps. The embedded sources have positions in agreement with those of the IRAS PSC, but some of the Herbig Ae/Be stars are offset significantly from the position of peak far-infrared emission. For all objects where it was possible to obtain 100 mu m flux densities, they are consistent with those observed by IRAS, but derived 50 mu m Aux densities are larger than expected. The far-infrared maps reveal that objects in at least 17 of 19 emission regions are significantly extended at the 30 "-40 " resolution of the Kuiper Airborne Observatory at 100 mu m. Only sources associated with AB Aur and possibly IRAS 05338-0624 have unresolved far-infrared emission. Detailed analyses of the flux densities and positions from our maps suggest the far-infrared emission in regions with Herbig Ae/Be stars may not immediately surround these stars in all cases. Instead, far-infrared emission from these objects may originate from dust heated externally by the Herbig stars, or from dust heated internally by other sources. For other objects arguably surrounded by far-infrared emission, the Herbig stars or embedded IRAS objects have similar mean deconvolved sizes (i.e., 0.10-0.15 pc), but possibly have different mean deconvolved shapes (i.e., aspect ratios). Thus, far-infrared emission here may originate from flattened dust envelopes; the appearance of a far-infrared object as either a Herbig Ae/Be star or an embedded IRAS source may be merely a matter of viewing orientation.
Low Abundance, Strong Features: Window-Dressing Crystalline Forsterite In The Disk Wall Of HD 100546
(2011-07) Mulders, G. D.; Waters, Lbfm; Dominik, C.; Sturm, B.; Bouwman, J.; Min, M.; Verhoeff, A. P.; Acke, B.; Augereau, J. C.; Evans, Neal J.; Henning, T.; Meeus, G.; Olofsson, J.; Evans, Neal J.
Context. Forsterite is one of the crystalline dust species that is often observed in protoplanetary disks and solar system comets. Being absent in the interstellar medium, it must be produced during the disk lifetime. It can therefore serve as a tracer of dust processing and disk evolution, which can lead to a better understanding of the physical processes occurring in the disk, and possibly planet formation. However, the connection of these processes with the overall disk crystallinity remains unclear. Aims. We aim to characterize the forsterite abundance and spatial distribution in the disk of the Herbig Be star HD 100546, to investigate if a connection exists with the large disk gap. Methods. We use a 2D radiative transfer code, MCMax, to model the circumstellar dust around HD 100546. We use VISIR Q-band imaging to probe the outer disk geometry and mid-infrared features to model the spatial distribution of forsterite. The temperature-dependent shape of the 69 mu m feature observed with Herschel***/PACS is used as a critical tool to constrain this distribution. Results. We find a crystalline mass fraction of 40-60%, located close to the disk wall between 13 and 20 AU, and possibly farther out at the disk surface. The forsterite is in thermal contact with the other dust species. We put an upper limit on the iron content of forsterite of 0.3%. Conclusions. Optical depth effects play a key role in explaining the observed forsterite features, hiding warm forsterite from view at short wavelengths. The disk wall acts as a showcase: it displays a localized high abundance of forsterite, which gives rise to a high observed crystallinity, while the overall mass fraction of forsterite is a factor of ten lower.
An N-Body Integrator For Gravitating Planetary Rings, And The Outer Edge Of Saturn's B Ring
(2013-08) Hahn, Joseph M.; Spitale, Joseph N.; Hahn, Joseph M.
A new symplectic N-body integrator is introduced, one designed to calculate the global 360. evolution of a self-gravitating planetary ring that is in orbit about an oblate planet. This freely available code is called epi_int, and it is distinct from other such codes in its use of streamlines to calculate the effects of ring self-gravity. The great advantage of this approach is that the perturbing forces arise from smooth wires of ring matter rather than discreet particles, so there is very little gravitational scattering and so only a modest number of particles are needed to simulate, say, the scalloped edge of a resonantly confined ring or the propagation of spiral density waves. The code is applied to the outer edge of Saturn's B ring, and a comparison of Cassini measurements of the ring's forced response to simulations of Mimas's resonant perturbations reveals that the B ring's surface density at its outer edge is sigma(0) = 195 +/- 60 g cm(-2), which, if the same everywhere across the ring, would mean that the B ring's mass is about 90% of Mimas's mass. Cassini observations show that the B ring-edge has several free normal modes, which are long-lived disturbances of the ring-edge that are not driven by any known satellite resonances. Although the mechanism that excites or sustains these normal modes is unknown, we can plant such a disturbance at a simulated ring's edge and find that these modes persist without any damping for more than similar to 10(5) orbits or similar to 100 yr despite the simulated ring's viscosity v(s) = 100 cm(2) s(-1). These simulations also indicate that impulsive disturbances at a ring can excite long-lived normal modes, which suggests that an impact in the recent past by perhaps a cloud of cometary debris might have excited these disturbances, which are quite common to many of Saturn's sharp-edged rings.
Observations of the stratorotational instability in rotating concentric cylinders
(2016-09) Ibanez, Ruy; Swinney, Harry L.; Rodenborn, Bruce; Ibanez, Ruy; Swinney, Harry L.
We study the stability of density stratified flow between corotating vertical cylinders with rotation rates ohm(o) < ohm(i) and radius ratio r(i)/r(o) = 0.877, where subscripts o and i refer to the outer and inner cylinders. Just as in stellar and planetary accretion disks, the flow has rotation, anticyclonic shear, and a stabilizing density gradient parallel to the rotation axis. The primary instability of the laminar state leads not to axisymmetric Taylor vortex flow but to a nonaxisymmetric stratorotational instability (SRI). The present work extends the range of Reynolds numbers and buoyancy frequencies [N = root(-g/rho)(d rho/dz)] examined in previous experiments. We present the first experimental results for the axial wavelength. of the instability as a function of the internal Froude number, Fr = ohm(i)/ N; lambda increases by nearly an order of magnitude over the range of Fr examined. For small outer cylinder Reynolds number, the SRI occurs for inner inner Reynolds number larger than for the axisymmetric Taylor vortex flow (i.e., the SRI is more stable). For somewhat larger outer Reynolds numbers the SRI occurs for smaller inner Reynolds numbers than Taylor vortex flow and even below the Rayleigh stability line for an inviscid fluid. Shalybkov and Rudiger [Astron. Astrophys. 438, 411 (2005)] proposed that the laminar state of a stably stratified rotating shear flow should be stable for ohm(o)/ohm(i) > r(i)/r(o), but we find that this stability criterion is violated for N sufficiently large. At large Reynolds number the primary instability is not the SRI but a previously unreported nonperiodic state that mixes the fluid.
Planet Formation in Circumbinary Configurations: Turbulence Inhibits Planetesimal Accretion
(2012-12) Meschiari, Stefano; Meschiari, Stefano
The existence of planets born in environments highly perturbed by a stellar companion represents a major challenge to the paradigm of planet Formation. In numerical simulations, the presence of a close binary companion stirs up the relative velocity between planetesimals, which is fundamental in determining the balance between accretion and erosion. However, the recent discovery of circumbinary planets by Kepler establishes that planet Formation in binary systems is clearly viable. We perform N-body simulations of planetesimals embedded in a protoplanetary disk, where planetesimal phasing is frustrated by the presence of stochastic torques, modeling the expected perturbations of turbulence driven by the magnetorotational instability. We examine perturbation amplitudes relevant to dead zones in the midplane (conducive to planet Formation in single stars), and find that planetesimal accretion can be inhibited even in the outer disk (4-10 AU) far from the central binary, a location previously thought to be a plausible starting point for the Formation of circumbinary planets.
Protostellar Disk Evolution Over Million-Year Timescales With A Prescription For Magnetized Turbulence
(2013-07) Landry, Russell; Dodson-Robinson, Sarah E.; Turner, Neal J.; Abram, Greg; Dodson-Robinson, Sarah E.
Magnetorotational instability (MRI) is the most promising mechanism behind accretion in low-mass protostellar disks. Here we present the first analysis of the global structure and evolution of non-ideal MRI-driven T-Tauri disks on million-year timescales. We accomplish this in a 1+1D simulation by calculating magnetic diffusivities and utilizing turbulence activity criteria to determine thermal structure and accretion rate without resorting to a three-dimensional magnetohydrodynamical (MHD) simulation. Our major findings are as follows. First, even for modest surface densities of just a few times the minimum-mass solar nebula, the dead zone encompasses the giant planet-forming region, preserving any compositional gradients. Second, the surface density of the active layer is nearly constant in time at roughly 10 g cm(-2), which we use to derive a simple prescription for viscous heating in MRI-active disks for those who wish to avoid detailed MHD computations. Furthermore, unlike a standard disk with constant-alpha viscosity, the disk midplane does not cool off over time, though the surface cools as the star evolves along the Hayashi track. Instead, the MRI may pile material in the dead zone, causing it to heat up over time. The ice line is firmly in the terrestrial planet-forming region throughout disk evolution and can move either inward or outward with time, depending on whether pileups form near the star. Finally, steady-Statemass transport is an extremely poor description of flow through an MRI-active disk, as we see both the turnaround in the accretion flow required by conservation of angular momentum and peaks in (M) over dot(R) bracketing each side of the dead zone. We caution that MRI activity is sensitive to many parameters, including stellar X-ray flux, grain size, gas/small grain mass ratio and magnetic field strength, and we have not performed an exhaustive parameter study here. Our 1+1D model also does not include azimuthal information, which prevents us from modeling the effects of Rossby waves.
Resolving Debris Discs In The Far-Infrared: Early Highlights From The DEBRIS Survey
(2010-07) Matthews, B. C.; Sibthorpe, B.; Kennedy, G.; Phillips, N.; Churcher, L.; Duchene, G.; Greaves, J. S.; Lestrade, J. F.; Moro-Martin, A.; Wyatt, M. C.; Bastien, P.; Biggs, A.; Bouvier, J.; Butner, H. M.; Dent, W. R. F.; Di Francesco, J.; Eisloffel, J.; Graham, J.; Harvey, P.; Hauschildt, P.; Holland, W. S.; Horner, J.; Ibar, E.; Ivison, R. J.; Johnstone, D.; Kalas, P.; Kavelaars, J.; Rodriguez, D.; Udry, S.; van der Werf, P.; Wilner, D.; Zuckerman, B.; Harvey, P. M.
We present results from the earliest observations of DEBRIS, a Herschel key programme to conduct a volume-and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 mu m are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.
The Role Of The Magnetorotational Instability In The Sun
(2014-05) Kagan, Daniel; Wheeler, J. Craig; Kagan, Daniel; Wheeler, J. Craig
We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes theta < 53 degrees. Nonaxisymmetric MRI modes grow faster than the corresponding axisymmetric modes; for some poloidal magnetic fields, the nonaxisymmetric MRI growth rates are similar to the angular rotation frequency Omega, while axisymmetric modes are stabilized. We briefly discuss the saturation of the field produced by MRI modes, finding that the implied field at the base of the convective zone in the Sun is comparable to that derived based on dynamos active in the tachocline and that the saturation of field resulting from the MRI may be of importance even in the upper convection zone.
The Spitzer c2d Survey Of Nearby Dense Cores. II. Discovery Of A Low-Luminosity Object In The "Evolved Starless Core" L1521F
(2006-09) Bourke, Tyler L.; Myers, Phillip C.; Evans, Neal J.; Dunham, Michael M.; Kauffmann, Jens; Shirley, Yancy L.; Crapsi, Antonio; Young, Chadwick H.; Huard, Tracy L.; Brooke, Tyler Y.; Chapman, Nicholas; Cieza, Lucas; Lee, Chang Won; Teuben, Peter; Wahhaj, Zahed; Dunham, Mike; Evans, Neal J.
We present Spitzer Space Telescope observations of the "evolved starless core" L1521F that reveal the presence of a very low luminosity object (L < 0.07 L-circle dot). The object, L1521F-IRS, is directly detected at mid- infrared wavelengths (> 5 mu m), but only in scattered light at shorter infrared wavelengths, showing a bipolar nebula oriented east- west that is probably tracing an outflow cavity. The nebula strongly suggests that L1521F-IRS is embedded in the L1521F core. Thus, L1521F-IRS is similar to the recently discovered L1014-IRS and the previously known IRAM 04191 in its substellar luminosity and dense core environment. However, these objects differ significantly in their core density, core chemistry, and outflow properties, and some may be destined to be brown dwarfs rather than stars.
A "Starless'' Core That Isn't: Detection of A Source in the L1014 Dense Core With the Spitzer Space Telescope
(2004-09) Young, Chadwick H.; Jorgensen, Jes K.; Shirley, Yancy L.; Kauffmann, Jens; Huard, Tracy; Lai, Shih-Ping; Lee, Chang Won; Crapsi, Antonio; Bourke, Tyler L.; Dullemond, Cornelis P.; Brooke, Timothy Y.; Porras, Alicia; Spiesman, William; Allen, Lori E.; Blake, Geoffrey A.; Evans, Neal J., II; Harvey, Paul M.; Koerner, David W.; Mundy, Lee G.; Myers, Philip C.; Padgett, Deborah L.; Sargent, Anneila I.; Stapelfeldt, Karl R.; van Dishoeck, Ewine F.; Bertoldi, Frank; Chapman, Nicholas; Cieza, Lucas; DeVries, Christopher H.; Ridge, Naomi A.; Wahhaj, Zahed; Young, Chadwick H.; Spiesman, William; Evans, Neal J., II; Harvey, Paul M.; Cieza, Lucas
We present observations of L1014, a dense core in the Cygnus region previously thought to be starless, but data from the Spitzer Space Telescope show the presence of an embedded source. We propose a model for this source that includes a cold core, heated by the interstellar radiation field, and a low-luminosity internal source. The low luminosity of the internal source suggests a substellar object. If L1014 is representative, other "starless'' cores may turn out to harbor central sources.
Supernova Asymmetries
(2007-10) Wheeler, J. C.; Maund, J. R.; Akiyama, S.; Wheeler, J. C.; Maund, J. R.
All core collapse supernovae are strongly aspherical. The >Bochum event> with velocity components displaced symmetrically about the principal H alpha line, strongly suggests that SN 1987A was a bi-polar rather than a uni-polar explosion. While there is a general tendency to display a single prominant axis in images and spectropolarimetry, there is also growing evidence for frequent departures from axisymmetry. There are various mechanisms that might contribute to large scale departures from spherical symmetry: jet-induced processes, the spherical shock accretion instability (SASI) and associated phenomena, and non-axisymmetric instabilities (NAXI). The MRI gives inevitable production of large toroidal magnetic fields. In sum: no Omega without B. The role of magnetic fields, non-axisymmetric instabilities, and of the de-leptonization phase are discussed.
The TEXES Survey For H-2 Emission From Protoplanetary Disks
(2008-12) Bitner, Martin A.; Richter, Matthew J.; Lacy, John H.; Herczeg, Gregory J.; Greathouse, Thomas K.; Jaffe, Daniel T.; Salyk, Colette; Blake, Geoffrey A.; Hollenbach, David J.; Doppmann, Greg W.; Najita, Joan R.; Currie, Thayne; Bitner, Martin A.; Richter, Matthew J.; Lacy, John H.; Herczeg, Gregory J.; Greathouse, Thomas K.; Jaffe, Daniel T.; Salyk, Colette; Blake, Geoffrey A.; Hollenbach, David J.; Doppmann, Greg W.; Najita, Joan R.; Currie, Thayne
We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H-2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H-2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H-2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (similar to 10 km s(-1)), centered at the stellar velocity, and spatially unresolved at scales of 0.4 '', which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures greater than or similar to 500 K from similar to 1 M-circle plus of gas. Our upper limits for the nondetections place upper limits on the amount of H-2 gas with T > 500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions that we are studying and that processes such as UV, X-ray, and accretion heating may be important.