arXiv subjects

J. C. Augereau

Publications and source records attributed to J. C. Augereau.

28 records · Page 2Linked to original sources

arXiv2007

Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks' to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation.

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arXiv2006

On the AU Mic debris disk: density profiles, grain properties and dust dynamics

We present the first comprehensive analysis of the AU Mic debris disk properties since the system was discovered by Kalas et al. (2004), and we explore whether the dynamical model, successful to reproduce the Beta Pic brightness profile could apply to AU Mic. We calculate the surface density profile of the AU Mic disk by performing the inversion of the near-IR and visible scattered light brightness profiles measured by Liu (2004a) and Krist et al. (2005), respectively. We discuss the grain properties by analysing the blue color of the disk in the visible (Krist et al. 2005) and by fitting the disk spectral energy distribution. We show that irrespective of the mean scattering asymmetry factor of the grains, most of the emission arises from an asymmetric, collisionally-dominated region that peaks close to the surface brightness break around 35 AU. The elementary scatterers at visible wavelengths are found to be sub-micronic, but the inferred size distribution underestimates the amount of large grains, resulting in too low sub-millimeter emissions compared to the observations. From our inversion procedure, we find that the V- to H-band scattering cross sections ratio increases outside 40 AU, in line with the observed color gradient of the disk. We show that a standard, solar-like stellar wind generates a pressure force onto the dust particles that behaves much like a radiation pressure force. With an assumed Mdot ~ 300 Mdot_sun, the wind pressure overcomes the radiation pressure and this effect is enhanced by the stellar flares. This explains the similarity between the Beta Pic and AU Mic brightness profiles. In both cases, the color gradient beyond 120 AU for Beta Pic and 35 AU for AU Mic, is believed to be a direct consequence of the dust dynamics.

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arXiv2004

Structures in Dusty Disks

Optically thin dusty disks around Main Sequence stars consist of debris from catastrophic collisions or from low erosion of long-lived planetesimals. Resolved observations of dusty disks have systematically evidenced asymmetries and annular structures. It has been proposed that some of these structures could be signatures of undetected exoplanets. In this paper, I review and discuss currently proposed models to account for the observed structures. These include the impact of distant exoplanets and/or of stellar companions. The Solar System serves as a reference case for these approaches and similarities are pointed out along the paper as well as some limitations of current modeling efforts.

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arXiv2003

Structuring the HD 141569 A circumstellar dust disk. Impact of eccentric bound stellar companions

Scattered light images of the optically thin dust disk around the 5 Myr old star HD141569 have revealed its complex asymmetric structure. We show in this paper that the surface density inferred from the observations presents similarities with that expected from a circumprimary disk within a highly eccentric binary system. We assume that either the two M stars in the close vicinity of HD141569 are bound companions or at least one of them is an isolated binary companion. We discuss the resulting interaction with an initially axisymmetric disk. This scenario accounts for the formation of a spiral structure, a wide gap in the disk and a broad faint extension outside the truncation radius of the disk after 10-15 orbital periods with no need for massive companion(s) in the midst of the disk resolved in scattered light. The simulations match the observations and the star age if the perturber is on an elliptic orbit with a periastron distance of 930 AU and an eccentricity from 0.7 to 0.9. We find that the numerical results can be reasonably well reproduced using an analytical approach proposed to explain the formation of a spiral structure by secular perturbation of a circumprimary disk by an external bound companion. We also interpret the redness of the disk in the visible reported by Clampin et al.(2003) and show that short-lived grains one order of magnitude smaller than the blow-out size limit are abundant in the disk. The most probable reason for this is that the disk sustains high collisional activity. Finally we conclude that additional processes are required to clear out the disk inside 150 AU and that interactions with planetary companions possibly coupled with the remnant gas disk are likely candidates.

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arXiv2002

Ground-based near-infrared imaging of the HD141569 circumstellar disk

We present the first ground-based near-infrared image of the circumstellar disk around the post-Herbig Ae/Be star HD141569A initially detected with the HST. Observations were carried out in the near-IR (2.2 $μ$m) at the Palomar 200-inch telescope using the adaptive optics system PALAO. The main large scale asymmetric features of the disk are detected on our ground-based data. In addition, we measured that the surface brightness of the disk is slightly different than that derived by HST observations (at 1.1 $μ$m and 1.6 $μ$m). We interpret this possible color-effect in terms of dust properties and derive a minimal

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arXiv2001

Dynamical modeling of large scale asymmetries in the beta Pictoris dust disk

We report a new and complete model of the beta Pictoris disk, which succeeds in accounting for both the surface brightness distribution, warp characteristics, the outer ``butterfly'' asymmetry as observed by HST/STIS in scattered light, as well as the infrared emission. Our model includes the presence of a disk of planetesimals extending out to 120-150 AU, perturbed gravitationally by a giant planet on an inclined orbit, following the approach of Mouillet et al. (1997b). At any time, the planetesimal disk is assumed to be the source of a distribution of grains produced through collisional evolution, with the same initial orbital parameter distribution. The steady state spatial grain distribution is found incorporating the effects of radiation pressure which can cause the distribution of the smallest particles to become very distended. With realistic assumptions about the grains' chemical properties, the modeling confirms the previously evident need for an additional population of hot grains close to the star, to account for the 12 microns fluxes at short distances from the star. It also indicates that this population cannot explain the outer 12 microns flux distribution when the effects of gravity and radiation pressure determine the distribution. Very small grains, produced by collisions among aggregates, are tentatively proposed to account for this 12 microns outer emission.

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arXiv2000

Dust disks around old Pre Main-Sequence stars: HST/NICMOS2 scattered light images and modeling

We present recent near-infrared detections of circumstellar disks around the two old PMS Herbig stars HD 141569 and HD 100546 obtained with the HST/NICMOS2 camera. They reveal extended structures larger than 350-400 AU in radius. While the HD 100546 disk appears as a continuous disk down to 40 AU, the HD 141569 environment seems more complex, splitted at least into two dust populations. As a convincing example, the full modeling of the disk surrounding HR 4796, another old PMS star, is detailed and confronted with more recent observations.

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arXiv2000

HST/NICMOS2 coronagraphic observations of the circumstellar environment of three old PMS stars: HD 100546, SAO 206462 and MWC 480

The close environment of four old Pre-Main Sequence stars has been observed thanks to the coronagraphic mode of the HST/NICMOS2 camera at lambda=1.6 micron. In the course of this program, the detection of a circumstellar annulus around HD 141569 has already been presented in Augereau et al.(1999b). In this paper, we report the detection of an elliptical structure around the Herbig Be star HD 100546 extending from the very close edge of the coronagraphic mask (~50 AU) to 350-380 AU (3.5-3.8 arcsec) from the star. The axis ratio gives a disk inclination of 51+/-3 degrees to the line-of-sight and a position angle of 161+/-5 degrees, measured east of north. At 50 AU, the disk has a surface brightness between 10.5 and 11 mag/arcsec^2, then follows a -2.92+/-0.04 radial power law up to 250-270 AU and finally falls as r^{-5.5+/-0.2}. The inferred optical thickness suggests that the disk is at least marginally optically thick inside 80 AU and optically thin further out. Combined with anisotropic scattering properties, this could explain the shape of a brightness asymmetry observed along the minor axis of the disk. This asymmetry needs to be confirmed. The circumstellar disks around SAO 206462 and MWC 480 are not resolved, leading to constraints on the dust distribution. A tight binary system separated by only 0.32+/-0.04 arcsec is nevertheless detected in the close vicinity of SAO 206462.

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arXiv1999

HST/NICMOS2 observations of the HD 141569 A circumstellar disk

We report the first resolved scattered light images of the circumstellar dust disk around the old Pre-Main Sequence star HD 141569 A. The disk seen in HST/NICMOS2 images shapes a bright annulus inclined at 37.5 degrees +/- 4.5 degrees from edge-on. This ring peaks at 325+/-10 AU from the star with a characteristic width of ~150 AU. At 1.6 microns, the dust grains scatter a total flux density of at least 4.5 +/- 0.5 mJy. Our disk model using the spatial distribution implied by the images does not explain the 10 microns excess and requires an additional grain population closer to the star. Some similarities and differences with the dust annulus surrounding HR 4796 A are pointed out.

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arXiv1999

On the HR 4796A circumstellar disk

We investigate in details the properties of the disk surrounding the 8 Myr old star HR 4796A, one of the few stars bringing precious clues to better understand the scenario which lead to planetary system formation. We propose a model able to reproduce all the available observations : the full spectral energy distribution from the mid-infrared to the millimeter wavelengths, resolved scattered light and thermal emission observations. We show that the circumstellar matter splits into two dust components : a cold annulus, peaked at 70 AU from the star, made of ISM-like grains (amorphous composition, porosity ~ 0.6) larger than 10 microns and a population of hot dust close to the star (at about 9 AU) made of comet-like grains (crystalline composition, porosity ~ 0.97). Both dust populations are highly collisional and the grain size distribution in the cold annulus is found to be cut-off by radiation pressure. At 70 AU, bodies as large as a few meters are required to fit the data leading to a minimum disk mass of a few Earth masses and to a gas to dust ratio less than 1. We discuss aftewards some implications on the disk structure and effects of larger bodies.

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