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Post Info TOPIC: Proplyds


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RE: Proplyds
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Title: Turmoil in Orion: The Nearest Massive Protostar
Authors: Jonathan C. Tan (Dept. of Astronomy, University of Florida)

I discuss different theories of massive star formation: formation from massive cores, competitive Bondi-Hoyle accretion, and protostellar collisions. I summarize basic features of the Turbulent Core Model (TCM). I then introduce the Orion Kleinmann-Low (KL) region, embedded in the Orion Nebula Cluster (ONC) and one of the nearest regions of massive star formation. The KL region contains three principal radio sources, known as "I", "n" and "BN". BN is known to be a runaway star, almost certainly set in motion by dynamical ejection within the ONC from a multiple system of massive stars, that would leave behind a recoiling, hard, massive, probably eccentric binary. I review the debate about whether this binary is Theta^1C, the most massive star in the ONC, or source "I", and argue that it is most likely to be Theta^1C, since this is now known be a recoiling, hard, massive, eccentric binary, with properties that satisfy the energy and momentum constraints implied by BN's motion. Source "n" is a relatively low-mass protostar with extended radio emission suggestive of a bipolar outflow. Source "I", located near the centre of the main gas concentration in the region, the Orion Hot Core, is the likely location of a massive protostar that is powering the KL region, and I discuss how its basic properties are consistent with predictions from the TCM. In this scenario, the radio emission from source "I" is the base of a bipolar outflow that is ionised by the massive protostar and should be elongated along the axis of the outflow.

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Debris discs
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Title: Outer edges of debris discs: how sharp is sharp?
Authors: Philippe Thebault, Yanqin Wu

Ring-like features have been observed in several debris discs. Outside the main ring, while some systems exhibit smooth surface brightness profiles (SB) that fall off roughly as r**-3.5, others display large luminosity drops at the ring's outer edge and steeper radial SB profiles. We seek to understand this diversity of outer edge profiles under the ``natural'' collisional evolution of the system, without invoking external agents such as planets or gas. We use a statistical code to follow the evolution of a collisional population, ranging from dust grains (submitted to radiation pressure) to planetesimals and initially confined within a belt (the 'birth ring'). The system typically evolves toward a "standard" steady state, with no sharp edge and SB \propto r**-3.5 outside the birth ring. Deviations from this standard profile, in the form of a sharp outer edge and a steeper fall-off, occur only when two parameters take their extreme values: 1) When the birth ring is so massive that it becomes radially optically thick for the smallest grains. However, the required disc mass is here probably too high to be realistic. 2) When the dynamical excitation of the dust-producing planetesimals is so low (<e> <0.01) that the smallest grains, which otherwise dominate the total optical depth, are preferentially depleted. This low-excitation case, although possibly not generic, cannot be ruled out by observations. Our "standard" profile provides a satisfactory explanation for a large group of debris discs with outer edges and SB falling as r**-3.5. Systems with sharper outer edges, barring other confining agents, could still be explained by ``natural'' collisional evolution if their dynamical excitation is very low. We show that such a dynamically-cold case provides a satisfactory fit for HR4796A

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Title: Disks around massive young stellar objects: are they common?
Authors: Zhibo Jiang, Motohide Tamura, Melvin G. Hoare, Yongqiang Yao, Miki Ishii, Min Fang, Ji Yang

We present K-band polarimetric images of several massive young stellar objects at resolutions ~ 0.1-0.5 arcsec. The polarisation vectors around these sources are nearly centro-symmetric, indicating they are dominating the illumination of each field. Three out of the four sources show elongated low-polarisation structures passing through the centres, suggesting the presence of polarisation disks. These structures and their surrounding reflection nebulae make up bipolar outflow/disk systems, supporting the collapse/accretion scenario as their low-mass siblings. In particular, S140 IRS1 show well defined outflow cavity walls and a polarisation disk which matches the direction of previously observed equatorial disk wind, thus confirming the polarisation disk is actually the circumstellar disk. To date, a dozen massive protostellar objects show evidence for the existence of disks; our work add additional samples around MYSOs equivalent to early B-type stars.

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Taurus-Auriga Class I/II protostars
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Title: Spitzer IRS Observations of Class I/II Objects in Taurus: Composition, Temperature and Thermal History of the Circumstellar Ices
Authors: G. Zasowski, F. Markwick-Kemper, Dan M. Watson, E. Furlan, C.J. Bohac, C. Hull, J.D. Green

We present observations of Taurus-Auriga Class I/II protostars obtained with the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15 micron features are made, using publicly-available laboratory data, to constrain the molecular composition, abundances, and levels of thermal processing along the lines of sight. We provide an inventory of the molecular environments observed, which have an average composition dominated by water ice with ~12% CO_2 (abundance relative to H_2O), >~2-9% CH_3OH, <~14% NH_3, ~4% CH_4, ~2% H_2CO, ~0.6% HCOOH, and ~0.5% SO_2. We find CO_2/H_2O ratios nearly equivalent to those observed in cold clouds and lines of sight toward the galactic centre. The unidentified 6.8 micron profile shapes vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH_4+ and CH_3OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermally-processed ice fractions and their implications for CO_2 formation and evolution. Comparison of our spectral fits to cold molecular cloud sight-lines indicate abundant prestellar ice environments made even richer by the radiative effects of protostars. Our results add additional constraints and a finer level of detail to current full-scale models of protostellar and protoplanetary systems.

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RE: Proplyds
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Title: On the Solar System-Debris Disk Connection
Authors: Amaya Moro-Martin

This paper emphasises the connection between solar and extra-solar debris disks: how models and observations of the Solar System are helping us understand the debris disk phenomenon, and vice versa, how debris disks are helping us place our Solar System into context.

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Class I Protostars
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Title: Spitzer IRS Spectra and Envelope Models of Class I Protostars in Taurus
Authors: E. Furlan (1,2), M. McClure (3), N. Calvet (4), L. Hartmann (4), P. D'Alessio (5), W. J. Forrest (3), D. M. Watson (3), K. I. Uchida (1), B. Sargent (3), J. D. Green (3), T. L. Herter (1) ((1) Cornell University, Ithaca, NY, (2) NAI/UCLA, Los Angeles, CA, (3) University of Rochester, Rochester, NY, (4) University of Michigan, Ann Arbor, MI, (5) UNAM, Morelia, Mexico)

We present Spitzer Infrared Spectrograph spectra of 28 Class I protostars in the Taurus star-forming region. The 5 to 36 micron spectra reveal excess emission from the inner regions of the envelope and accretion disk surrounding these predecessors of low-mass stars, as well as absorption features due to silicates and ices. Together with shorter- and longer-wavelength data from the literature, we construct spectral energy distributions and fit envelope models to 22 protostars of our sample, most of which are well-constrained due to the availability of the IRS spectra. We infer that the envelopes of the Class I objects in our sample cover a wide range in parameter space, particularly in density and centrifugal radius, implying different initial conditions for the collapse of protostellar cores.

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Debris Disks
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Title: Long-Term Collisional Evolution of Debris Disks
Authors: Torsten Löhne, Alexander V. Krivov, Jens Rodmann
(Version v2)

We simulated the long-term collisional depletion of debris disks around solar-type (G2V) stars with our code. The numerical results were supplemented by, and interpreted through, a new analytic model. A few general scaling rules for the disk evolution are suggested. The timescale of the collisional evolution is inversely proportional to the initial disk mass and scales with radial distance as r^4.3 and with eccentricities of planetesimals as e^-2.3. Further, we show that at actual ages of debris disks between 10 Myr and 10 Gyr, the decay of the dust mass and the total disk mass follow different laws. The reason is that the collisional lifetime of planetesimals is size-dependent. At any moment, there exists a transitional size, which separates larger objects that still have the ''primordial'' size distribution set in the growth phase from small objects whose size distribution is already set by disruptive collisions. The dust mass and its decay rate evolve as that transition affects objects of ever-larger sizes. Under standard assumptions, the dust mass, fractional luminosity, and thermal fluxes all decrease as t^xi with xi = -0.3...-0.4. Specific decay laws of the total disk mass and the dust mass, including the value of xi, largely depend on a few model parameters, such as the critical fragmentation energy as a function of size, the primordial size distribution of largest planetesimals, as well as the characteristic eccentricity and inclination of their orbits. With standard material prescriptions and a distribution of disk masses and extents, a synthetic population of disks generated with our analytic model agrees quite well with the observed Spitzer/MIPS statistics of 24 and 70 micron fluxes and colours versus age.

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Astronomers at the University of Rochester are pointing to three nearby stars they say may hold "embryonic planets"a missing link in planet-formation theories.
As scientists try to piece together how our own planet came to be, they look to the forming planets of other star systems for clues. But astronomers have been unable to find evidence for one of the key stages of planet development, a period early in the planet's formation when it is only as large as tiny Pluto.

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Puffy debris disks around three nearby stars could harbour Pluto-sized planets-to-be, a new computer model suggests.
The "planet embryos" are predicted to orbit three young, nearby stars, located within about 60 light years or less of our solar system. AU Microscopii and Beta Pictoris are both estimated to be about 12 million years old, while a third star, Fomalhaut, is aged at 200 million years old.
If confirmed, the objects would represent the first evidence of a never-before-observed stage of early planet formation. Another team recently spotted "space lint" around a nearby star that pointed to an even earlier phase of planet building, when baseball-sized clumps of interstellar dust grains are colliding together.

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V2129 Ophiuchi.kmz
Google Sky file

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