* Astronomy

Members Login
Post Info TOPIC: FU Orionis


L

Posts: 131433
Date:
RE: FU Orionis
Permalink  
 


Title: The Early ALMA View of the FU Ori Outburst System
Author: A. S. Hales, S. A. Corder, W. R. D. Dent, S. M. Andrews, J. A. Eisner, L.A. Cieza

We have obtained ALMA Band 7 observations of the FU Ori outburst system at 0.6"x0.5" resolution to measure the link between the inner disk instability and the outer disk through sub-mm continuum and molecular line observations. Our observations detect continuum emission which can be well modelled by two unresolved sources located at the position of each binary component. The interferometric observations recover the entire flux reported in previous single-dish studies, ruling out the presence of a large envelope.

Read more (4207kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Protoplanetary dust porosity and FU Orionis Outbursts: Solving the mystery of Earth's missing volatiles
Author: Alexander Hubbard, Denton S. Ebel

The Earth is known to be depleted in volatile lithophile elements in a fashion that defies easy explanation. We resolve this anomaly with a model that combines the porosity of collisionally grown dust grains in protoplanetary disks with heating from FU Orionis events that dramatically raise protoplanetary disk temperatures. The heating from an FU Orionis event alters the aerodynamical properties of the dust while evaporating the volatiles. This causes the dust to settle, abandoning those volatiles. The success of this model in explaining the elemental composition of the Earth is a strong argument in favor of highly porous collisionally grown dust grains in protoplanetary disks outside our Solar System. Further, it demonstrates how thermal (or condensation based) alterations of dust porosity, and hence aerodynamics, can be a strong factor in planet formation, leading to the onset of rapid gravitational instabilities in the dust disk and the subsequent collapse that forms planetesimals.

Read more (516kb, PDF)



__________________


L

Posts: 131433
Date:
FU Orionis binary system
Permalink  
 


Title: Constraining mass ratio and extinction in the FU Orionis binary system with infrared integral field spectroscopy
Authors: Laurent Pueyo, Lynne Hillenbrand, Gautam Vasisht, Ben R. Oppenheimer, John D. Monnier, Sasha Hinkley, Justin Crepp, Lewis C. Roberts Jr, Douglas Brenner, Neil Zimmerman, Ian Parry, Charles Beichman, Richard Dekany, Mike Shao, Rick Burruss, Eric Cady, Jenny Roberts, Remi Soummer

We report low resolution near infrared spectroscopic observations of the eruptive star FU Orionis using the Integral Field Spectrograph Project 1640 installed at the Palomar Hale telescope. This work focuses on elucidating the nature of the faint source, located 0.5" south of FU Ori, and identified in 2003 as FU Ori S. We first use our observations in conjunction with published data to demonstrate that the two stars are indeed physically associated and form a true binary pair. We then proceed to extract J and H band spectro-photometry using the damped LOCI algorithm, a reduction method tailored for high contrast science with IFS. This is the first communication reporting the high accuracy of this technique, pioneered by the Project 1640 team, on a faint astronomical source. We use our low resolution near infrared spectrum in conjunction with 10.2 micron interferometric data to constrain the infrared excess of FU Ori S. We then focus on estimating the bulk physical properties of FU Ori S. Our models lead to estimates of an object heavily reddened, A_V =8-12, with an effective temperature of ~ 4000-6500 K . Finally we put these results in the context of the FU Ori N-S system and argue that our analysis provides evidence that FU Ori S might be the more massive component of this binary system

Read more (1373kb, PDF)



__________________


L

Posts: 131433
Date:
RE: FU Orionis
Permalink  
 


Title: The Periodic Spectroscopic Variability of FU Orionis
Authors: Stacie L. Powell, Mike Irwin, Jerome Bouvier, Cathie J. Clarke

FU Orionis systems are young stars undergoing outbursts of disc accretion and where the optical spectrum contains lines associated with both the disc photosphere and a wind component. Previous observations of the prototype FU Orionis have suggested that the wind lines and the photospheric lines are modulated with periods of 14.54 and 3.54 days respectively (Herbig et al. 2003). We have re-observed the system at higher spectral resolution, by monitoring variations of optical line profiles over 21 nights in 2007 and have found periods of 13.48 and 3.6 days in the wind and disc components consistent with the above: this implies variability mechanisms that are stable over at least a decade. In addition we have found: i) that the variations in the photospheric absorption lines are confined to the blue wing of the line (around -9km/s): we tentatively ascribe this to an orbiting hotspot in the disc which is obscured by a disc warp during its receding phase. ii) The wind period is manifested not only in blue-shifted Halpha absorption, but also in red-shifted emission of Halpha and Hbeta, as well as in blue-shifted absorption of Na I D, Li I and Fe II. iii) We find that the periodic modulation of blue-shifted Halpha absorption at around -100km/s, is phase lagged with respect to variations in the other lines by ~1.8days. This is consistent with a picture in which variations at the wind base first affect chromospheric emission and then low velocity blue-shifted absorption, followed - after a lag equal to the propagation time of disturbances across the wind's acceleration region - by a response in high velocity blue-shifted absorption. Such arguments constrain the size of the acceleration region to ~10^12cm. We discuss possible mechanisms for periodic variations within the innermost 0.1AU of the disc, including the possibility that these variations indicate the presence of an embedded hot Jupiter.

Read more (2542kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Dead Zones around Young Stellar Objects: FU Orionis Outbursts and Transition Discs
Authors: Rebecca G. Martin, Stephen H. Lubow, Mario Livio, J. E. Pringle

We perform global time-dependent simulations of an accretion disc around a young stellar object with a dead zone (a region where the magneto-rotational instability cannot drive turbulence because the material is not sufficiently ionised). For infall accretion rates on to the disc of around 10^-7 Msun/yr, dead zones occur if the critical magnetic Reynolds number is larger than about 10^4. We model the collapse of a molecular gas cloud. At early times when the infall accretion rate is high, the disc is thermally ionised and fully turbulent. However, as the infall accretion rate drops, a dead zone may form if the critical magnetic Reynolds number is sufficiently large, otherwise the disc remains fully turbulent. With a dead zone the disc can become unstable to the gravo-magneto instability. The mass of the star grows in large accretion outbursts that may explain FU Orionis events. At late times there is not sufficient mass in the disc for outbursts to occur but the dead zone becomes even more prominent as the disc cools. Large inner dead zones in the later stages of disc evolution may help to explain observations of transition discs with an inner hole.

Read more  (70kb, PDF) 



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: 2-D simulations of FU Orionis disk outbursts
Authors: Zhaohuan Zhu, Lee Hartmann, Charles Gammie, Jonathan C. McKinney

We have developed time-dependent models of FU Ori accretion outbursts to explore the physical properties of protostellar disks. Our two-dimensional, axisymmetric models incorporate full vertical structure with a new treatment of the radiative boundary condition for the disk photosphere. We find that FU Ori-type outbursts can be explained by a slow accumulation of matter due to gravitational instability. Eventually this triggers the magnetorotational instability, which leads to rapid accretion. The thermal instability is triggered in the inner disk but this instability is not necessary for the outburst. An accurate disk vertical structure, including convection, is important for understanding the outburst behaviour. Large convective eddies develop during the high state in the inner disk. The models are in agreement with Spitzer IRS spectra and also with peak accretion rates and decay timescales of observed outbursts, though some objects show faster rise timescale. We also propose that convection may account for the observed mild-supersonic turbulence and the short-timescale variations of FU Orionis objects.

Read more  (2115kb, PDF)

__________________


L

Posts: 131433
Date:
FU Orionis objects
Permalink  
 


Title: Long-wavelength excesses of FU Orionis objects: flared outer disks or infalling envelopes?
Authors: Zhaohuan Zhu, Lee Hartmann, Nuria Calvet, Jesus Hernandez, Ajay-Kumar Tannirkulam, Paola D'Alessio

The mid- to far-infrared emission of the outbursting FU Orionis objects has been attributed either to a flared outer disk or to an infalling envelope. We revisit this issue using detailed radiative transfer calculations to model the recent, high signal-to-noise data from the IRS instrument on the Spitzer Space Telescope. In the case of FU Ori, we find that a physically-plausible flared disk irradiated by the central accretion disk matches the observations. Building on our previous work, our accretion disk model with outer disk irradiation by the inner disk reproduces the spectral energy distribution between ~4000 angstroms to ~40 microns. Our model is consistent with near-infrared interferometry but there are some inconsistencies with mid-infared interferometric results. Including the outer disk allows us to refine our estimate of the outer radius of the outbursting, high mass accretion rate disk in FU Ori as ~ 0.5 AU, which is a crucial parameter in assessing theories of the FU Orionis phenomenon. We are able to place an upper limit on the mass infall rate of any remnant envelope infall rate to ~ 7e-7 Msun/yr assuming a centrifugal radius of 200 AU. The FUor BBW 76 is also well modelled by a 0.6 AU inner disk and a flared outer disk. However, V1515 Cyg requires an envelope with an outflow cavity to adequately reproduce the IRS spectrum. In contrast with the suggestion by Green et al., we do not require a flattened envelope to match the observations; the inferred cavity shape is qualitatively consistent with typical protostellar envelopes. This variety of dusty structures suggests that the FU Orionis phase can be present at either early or late stages of protostellar evolution.

Read more (298kb, PDF)

__________________


L

Posts: 131433
Date:
RE: FU Orionis
Permalink  
 


Title: High Resolution Near-Infrared Spectroscopy of FUors and FUor-like stars
Authors: Tom Greene, Colin Aspin, Bo Reipurth

We present new high resolution (R=18,000) near-infrared spectroscopic observations of a sample of classical FU Orionis stars (FUors) and other young stars with FUor characteristics that are sources of Herbig-Haro flows. Spectra are presented for the region 2.203 - 2.236 microns which is rich in absorption lines sensitive to both effective temperatures and surface gravities of stars. Both FUors and FUor-like stars show numerous broad and weak unidentified spectral features in this region. Spectra of the 2.280 - 2.300 micron region are also presented, with the 2.2935 micron v=2-0 CO absorption bandhead being clearly the strongest feature seen in the spectra all FUors and Fuor-like stars. A cross-correlation analysis shows that FUor and FUor-like spectra in the 2.203 - 2.236 micron region are not consistent with late-type dwarfs, giants, nor embedded protostars. The cross-correlations also show that the observed FUor-like Herbig-Haro energy sources have spectra that are substantively similar to those of FUors. Both object groups also have similar near-infrared colours. The large line widths and double-peaked nature of the spectra of the FUor-like stars are consistent with the established accretion disk model for FUors, also consistent with their near-infrared colours. It appears that young stars with FUor-like characteristics may be more common than projected from the relatively few known classical FUors.

Read more (68kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Evolution of dust and ice features around FU Orionis objects
Authors: S. P. Quanz, Th. Henning, J. Bouwman, R. van Boekel, A. Juhasz, H. Linz (all Max Planck Institute for Astronomy, Heidelberg/Germany), K.M. Pontoppidan (California Institute of Technology, Pasadena/USA), F. Lahuis (Leiden Observatory, Leiden/Netherlands; SRON Netherlands Institute for Space Research, Groningen/Netherlands)

We present spectroscopy data for a sample of 14 FUors and 2 TTauri stars observed with the Spitzer Space Telescope or with the Infrared Space Observatory (ISO). Based on the appearance of the 10 micron silicate feature we define 2 categories of FUors. Objects showing the silicate feature in absorption (Category 1) are still embedded in a dusty and icy envelope. The shape of the 10 micron silicate absorption bands is compared to typical dust compositions of the interstellar medium and found to be in general agreement. Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but a superposed emission feature can explain the observed shape. We derive optical depths and extinction values from the silicate band and additional ice bands at 6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at 15.2 micron allows us to search for evidence for ice processing and constrains whether the absorbing material is physically linked to the central object or in the foreground. For objects showing the silicate feature in emission (Category 2), we argue that the emission comes from the surface layer of accretion disks. Analysing the dust composition reveals that significant grain growth has already taken place within the accretion disks, but no clear indications for crystallisation are present. We discuss how these observational results can be explained in the picture of a young, and highly active accretion disk. Finally, a framework is proposed as to how the two categories of FUors can be understood in a general paradigm of the evolution of young, low-mass stars. Only one object (Parsamian 21) shows PAH emission features. Their shapes, however, are often seen toward evolved stars and we question the object's status as a FUor and discuss other possible classifications.

Read more (766kb, PDF)


__________________


L

Posts: 131433
Date:
ESPaDOnS
Permalink  
 


The Canada-France-Hawaii Telescope (CFHT), one of the oldest facilities on the summit of Mauna-Kea, is at the forefront of optical astronomy through stunning wide field images of the universe, thanks to MegaPrime, the largest digital camera in operation on a telescope.

However, for the past months, a new instrument nick-named ESPaDOnS has shared the telescope with MegaPrime to look at stars in a search of their magnetic field.
This new instrument is now offering a unique opportunity to study stars through the observation of their magnetic activity.



Jean-Francois Donati, who led the construction of ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) before its delivery to CFHT, and his colleagues used the instrument to study visible light coming from the FU Orionis system, which contains a young star that is being fed by a surrounding disk. The magnetic field inside this disk helps to slow its rotation, causing material to fall towards the central star. Although theoretical models predict this crucial role, the magnetic field close to the star has not hitherto been directly measured.

Light from FU Orionis is rotated, or polarised, by magnetic fields in the disk. By measuring this polarization, the astronomers found that the magnetic field slows down the disk material much more than models predict. This may explain why the star does not spray out some of the in-falling material as a jet, a feature seen in other star-disk systems.

Source

__________________
1 2  >  Last»  | Page of 2  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard