arXiv subjects

C. P. Dullemond

Publications and source records attributed to C. P. Dullemond.

At least 127 records · Page 7Linked to original sources

arXiv2005

Evaporation of ion-irradiated disks

We calculate the evaporation of a cool accretion disk around a black hole due to the ion-bombardment by an ion supported accretion flow (here ISAF, or optically thin ADAF). As first suggested by Spruit & Deufel (2002), this evaporation takes place in two stages: ion bombardment of the cool disk (Shakura-Sunyaev disk: SSD) produces an intermediate-temperature layer on top of the disk (`warm layer') which constitutes an independent accretion flow on both sides of the SSD. As this warm material accretes inward of the inner radius of the SSD, it becomes thermally unstable by lack of cooling of photons, and evaporates into the ISAF, thereby feeding the latter. Angular momentum conservation forces a certain fraction of the ISAF material to move outward, where it can bombard the SSD with its hot ions. The flow geometry is derived by computing stationary solutions of the continuity- and angular momentum equations for the three components (ISAF, warm flow and SSD). The overall radiative output is dominated by hard X-rays. They are produced mostly from the warm component, rather than the ISAF. The expected time dependence and stability of the flow, not computed here, is discussed briefly.

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arXiv2004

Infrared emission in radio galaxy NGC 4261

We have analyzed the total and nuclear SED for NGC 4261 and find that the dominant process for the mid- and far-infrared emission in this object is non-thermal emission from the active nucleus. Modeling the emission from the optically detected 300 pc dust disk yields no significant disk contribution at any wavelength. To explain the observations, either the disk has an inflated inner region which partly absorbs the core, or the intrinsic core spectrum is curved. The inner 10 pc of the disk can potentially be conceived as an obscuring torus, albeit with optical depth around unity.

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arXiv2004

Dust coagulation in protoplanetary disks: a rapid depletion of small grains

We model the process of dust coagulation in protoplanetary disks and calculate how it affects their observational appearance. Our model involves the detailed solution of the coagulation equation at every location in the disk. At regular time intervals we feed the resulting 3-D dust distribution functions into a continuum radiative transfer code to obtain spectral energy distributions. We find that, even if only the very basic -- and well understood -- coagulation mechanisms are included, the process of grain growth is much too quick to be consistent with infrared observations of T Tauri disks. Small grains are removed so efficiently that, long before the disk reaches an age of 1E6 years typical of T Tauri stars, the SED shows only very weak infrared excess. This is inconsistent with observed SEDs of most classical T Tauri stars. Small grains somehow need to be replenished, for instance by aggregate fragmentation through high-speed collisions. A very simplified calculation shows that when aggregate fragmentation is included, a quasi-stationary grain size distribution is obtained in which growth and fragmentation are in equilibrium. This quasi-stationary state may last 1E6 years or even longer, dependent on the circumstances in the disk, and may bring the time scales into the right regime. We conclude that a simple evolutionary scenario in which grains slowly grow from pristine 0.1 micron grains to larger grains over a period of a few Myr is most likely incorrect.

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arXiv2004

A submillimeter exponential disk in M~51: evidence for an extended cold dust disk

A 850 micron map of the interacting spiral galaxy M51 shows well-defined spiral arms, closely resembling the structures seen in CO and HI emission. However, most of the 850 micron emission originates in an underlying exponential disk, a component that has not been observed before in a face-on galaxy at these wavelengths. The scale-length of this disk is 5.45 kpc, which is somewhat larger than the scale-length of the stellar disk, but somewhat smaller than that of atomic hydrogen. Its profile can not be explained solely by a radial disk temperature gradient but requires the underlying dust to have an exponential distribution as well. This reinforces the view that the submm emission from spiral galaxy disks traces total hydrogen column density, i.e.the sum of H2 and HI. A canonical gas-to-dust ratio of 100+/-26 is obtained for kappa(850)=1.2 g**-1 cm**2, where kappa(850) is the dust opacity at 850 micron.

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arXiv2004

Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4

We present accurate mid-infrared observations of the disk around the young, bona-fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in the 7.9, 10.4 and 12.3 micron filters, from which we infer the presence of a prominent, broad silicate emission feature. The shape of the silicate feature is dominated by emission from 2 micron amorphous olivine grains. Such grains, being an order of magnitude larger than those in the interstellar medium, are a first proof of dust processing and grain growth in disks around brown dwarfs. The object's spectral energy distribution is below the prediction of the classical flared disk model but higher than that of the two-layer flat disk. A good match can be achieved by using an intermediate disk model with strongly reduced but non-zero flaring. Grain growth and dust settling processes provide a natural explanation for this disk geometry and we argue that such intermediate flaring might explain the observations of several other brown dwarf disks as well.

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arXiv2004

The gas temperature in the surface layers of protoplanetary disks

Models for the structure of protoplanetary disks have so far been based on the assumption that the gas and the dust temperature are equal. The gas temperature, an essential ingredient in the equations of hydrostatic equilibrium of the disk, is then determined from a continuum radiative transfer calculation, in which the continuum opacity is provided by the dust. It has been long debated whether this assumption still holds in the surface layers of the disk, where the dust infrared emission features are produced. In this paper we compute the temperature of the gas in the surface layers of the disk in a self-consistent manner. The gas temperature is determined from a heating-cooling balance equation in which processes such as photoelectric heating, dissociative heating, dust-gas thermal heat exchange and line cooling are included. The abundances of the dominant cooling species such as CO, C, C+ and O are determined from a chemical network based on the atomic species H, He, C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find that in general the dust and gas temperature are equal to withing 10% for A_V >~ 0.1, which is above the location of the `super-heated surface layer' in which the dust emission features are produced (e.g. Chiang & Goldreich 1997). High above the disk surface the gas temperature exceeds the dust temperature and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as high as 600 K at a radius of 100 AU. This is a region where CO has fully dissociated, but a significant fraction of hydrogen is still in molecular form. The densities are still high enough for non-negligible H_2 emission to be produced.....(see paper for full abstract)

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arXiv2004

[O I] 6300 A Emission from Disks Around Herbig Ae/Be Stars

We present high spectral-resolution optical spectra of 49 Herbig Ae/Be stars in a search for the [O I] 6300.2 A line. The vast majority of the stars in our sample show narrow (FWHM < 100 km/s) emission lines, centered on the stellar radial velocity. Some stars in our sample show double-peaked lines profiles, with peak-to-peak separations of about 10 km/s. The presence and strength of the [O I] line emission appears to be well correlated with the far-infrared energy distribution of each source: stars with a strong excess at 60 micron have in general stronger [O I] emission than stars with weaker 60 micron excesses. We interpret the observed [O I] 6300.2 A line profiles as arising in the surface layers of the protoplanetary disks surrounding Herbig Ae/Be stars.

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arXiv2004

A "Starless" Core that Isn't: Detection of a Source in the L1014 Dense Core with the Spitzer Space Telescope

We present observations of L1014, a dense core in the Cygnus region previously thought to be starless, but data from the Spitzer Space Telescope shows 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.

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arXiv2004

The effect of dust settling on the appearance of protoplanetary disks

We analyze how the process of dust settling affects the spectral energy distribution and optical appearance of protoplanetary disks. Using simple analytic estimates on the one hand, and detailed 1+1-D models on the other hand, we show that, while the time scale for settling down to the equator may exceed the life time of the disk, it takes much less time for even small grains of 0.1 micron to settle down to a few pressure scale heights. This is often well below the original location of the disk's photosphere, and the disk therefore becomes effectively 'flatter'. If turbulent stirring is included, a steady state solution can be found, which is typically reached after a few times 1E5 years. Dependent on the strength of the turbulence, the shape of the disk in such a steady state can be either fully flaring, or flaring only up to a certain radius and self-shadowed beyond that radius. We show that these partly self-shadowed disks have a much weaker mid- to far-infrared flux than the fully flaring ones. We also show that these self-shadowed regions of the disk are very weak in resolved images of scattered light, in contrast to the fully flaring disks. From the calculations with compact grains it follows that, after about 1E6 years, most disks should be self-shadowed. The fact that some older disks are still observed with the characteristics of flaring disks therefore seems somewhat inconsistent with the time scales predicted by the settling model based on compact grains. This suggests that perhaps even the small grains (lesssim 0.1 micron) have a porous or fractal structure, or that the different geometries of observed disks is merely a reflection of the turbulent state of these disks.

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arXiv2004

Correlation between grain growth and disk geometry in Herbig Ae/Be systems

We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Be stars, for which we can determine the infrared spectral energy distribution (SED). We find a clear correlation between the strength of the ratio of the near- to mid-infrared excess of these sources, and the slope of the (sub-)mm energy distribution. Based on earlier multi-dimensional modeling of disks around Herbig Ae stars, we interpret this as a correlation between the geometry of the disk (flared or self-shadowed) and the size of the grains: self-shadowed disks have, on average, larger grains than their flared counterparts. These data suggest that the geometry of a young stellar disk evolves from flared to self-shadowed.

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arXiv2004

The 2D Continuum Radiative Transfer Problem: Benchmark Results for Disk Configurations

We present benchmark problems and solutions for the continuum radiative transfer (RT) in a 2D disk configuration. The reliability of three Monte-Carlo and two grid-based codes is tested by comparing their results for a set of well-defined cases which differ for optical depth and viewing angle. For all the configurations, the overall shape of the resulting temperature and spectral energy distribution is well reproduced. The solutions we provide can be used for the verification of other RT codes.We also point out the advantages and disadvantages of the various numerical techniques applied to solve the RT problem.

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arXiv2004

Flaring vs. self-shadowed disks: the SEDs of Herbig Ae/Be stars

Isolated Herbig Ae stars can be divided into two groups (Meeus et al. 2001): those with an almost flat spectral energy distribution in the mid-infrared (`group I'), and those with a strong decline towards the far-infrared (`group II'). In this paper we show that the group I vs. II distinction can be understood as arising from flaring vs. self-shadowed disks. We show that these two types of disks are natural solutions of the 2-D radiation-hydrostatic structure equations. Disks with high optical depth turn out to be flaring and have a strong far-IR emission, while disks with an optical depth below a certain threshold drop into the shadow of their own puffed-up inner rim and are weak in the far-IR. In spite of not having a directly irradiated surface layer, self-shadowed disks still display dust features in emission, in agreement with observations of group II sources. We propose an evolutionary scenario in which a disk starts out with a flaring shape (group I source), and then goes through the process of grain growth, causing the optical depth of the disk to drop and the disk to become self-shadowed (group II source). We show that this scenario predicts that the (sub-)millimeter slope of the disk changes from steep (small grains) to Rayleigh-Jeans-like (large grains) in the early stages of evolution, so that all group II sources are expected to have Rayleigh-Jeans-like slopes, while some group I sources may still have steep (sub-)millimeter slopes.

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arXiv2003

Explaining UXOR variability with self-shadowed disks

In this Letter we propose a new view on UX Orionis type variability. The idea is based on the earlier proposal by various authors that UXORs are nearly-edge-on disks in which hydrodynamic fluctuations could cause clumps of dust and gas to cross the line of sight. However, because the standard disk models have a flaring geometry, it is mostly the outer regions of the disk that obscure the star. The time scales for such obscuration events would be too long to match the observed time scales of weeks to months. Recent 2-D self-consistent models of Herbig Ae/Be protoplanetary disks (Dullemond et al. 2002,2003 henceforth D02/DD03), however, have indicated that for Herbig Ae/Be star disks there exists, in addition to the usual flared disks, also a new class of disks: those that are fully self-shadowed. Only their puffed-up inner rim (at the dust evaporation radius) is directly irradiated by the star, while the disk at larger radius resides in the shadow of the rim. For these disks there exist inclinations at which the line of sight towards the star skims the upper parts of the puffed-up inner rim, while passing high over the surface of outer disk regions. Small hydrodynamic fluctuations in the puffed-up inner rim could then be held responsible for the extinction events seen in UXORs. If this idea is correct, it makes a prediction for the shape of the SEDs of these stars. It was shown by D02/DD03 that flared disks have a strong far-IR excess and can be classified as `group I' (in the classification of Meeus et al. 2001), while self-shadowed disks have a relatively weak far-IR excess and are classified as `group II'. Our model therefore predicts that UXORs belong to the `group II' sources. We show that this correlation is indeed found within a sample of 86 Herbig Ae/Be stars.

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arXiv2003

The effect of scattering on the structure and SED of protoplanetary disks

In this paper we investigate how the inclusion of scattering of the stellar radiation into a passive flaring disk model affects its structure and spectral energy distribution, and whether neglecting it could significantly decrease the model reliability. In order to address these questions we construct a detailed 1+1D vertical structure model in which the scattering properties of the dust can be varied. Models are presented with and without dust scattering, and for different albedos and phase functions. It is found that scattering has the effect of reducing the disk temperature at all heights, so that the disk "shrinks", i.e., the the density at all intermediate heights decreases. However, this effect in most cases is more than compensated by the increase of the total extinction (absorption + scattering) cross section, so that the surface scale height increases, and images in scattered light will see a slightly thicker disk. The integrated infrared emission decreases as the albedo increases, because an increasing part of the flux captured by the disk is reflected away instead of absorbed and reprocessed. The reduction of the infrared thermal emission of the disk is stronger at short wavelengths (near infrared) and practically negligible at millimeter wavelengths. For relatively low albedo (alb <~ 0.5), or for strongly forward-peaked scattering (g roughly >0.8), the infrared flux reduction is relatively small.

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arXiv2003

The First Detailed Look at a Brown Dwarf Disk

The combination of mid-infrared and recent submm/mm measurements allows us to set up the first comprehensive spectral energy distribution (SED) of the circumstellar material around a young Brown Dwarf. Simple arguments suggest that the dust is distributed in the form of a disk. We compare basic models to explore the disk parameters. The modeling shows that a flat disk geometry fits well the observations. A flared disk explains the SED only if it has a puffed-up inner rim and an inner gap much larger than the dust sublimation radius. Similarities and differences with disks around T Tauri stars are discussed.

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arXiv2003

An analysis of two-layer models for circumstellar disks

The two-layer disk models of Chiang & Goldreich (1997, henceforth CG) and its derivatives are popular among astronomers because of their simplicity and the clear predictions they make for the SEDs of T Tauri stars and Herbig Ae/Be stars. Moreover, they can be computed quickly, which is a great advantage when fitting observations using automated procedures. In this paper we wish to assess the accuracy and reliability of 2-layer models, by comparing them to detailed vertical structure models with accurate 1+1D radiative transfer. We focus on the shape of the SED, and the predicted height and "flaring index" of the disk. We first consider models where scattering is set to zero. We find that 2-layer models overestimate significantly the near-infrared flux, and we suggest a simple way of correcting this effect, at least in part. At longer wavelengths, the SED of two-layer models often show a two-bump structure, which is absent in 1+1D models. Nevertheless, overall agreement is reasonably good, and the differences are in most cases within 30%. At (sub)-mm wavelengths the differences may even be less. The shape of the disk, as measured by its pressure and surface scale height and by the flaring angle are also well reproduced by two-layer models. When scattering is included in the 1+1D models, the differences become larger, especially in the near-infrared. We suggest simple ways to include scattering in two-layer models and discuss their reliability. We do not compare the two-layer models to full 2-D/3-D models, so the conclusions remain valid only within the annulus-by-annulus approximation.

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arXiv2003

New radiative transfer models for obscuring tori in active galaxies

Two-dimensional radiative transfer is employed to obtain the broad-band infrared spectrum of active galaxies. In the models we vary the geometry and size of the obscuring medium, the surface density, the opacity and the grain size distribution. Resulting spectral energy distributions are constructed for different orientations of the toroid. Colour-colour comparisons with observational data are consistent with previous observations that the emission longward of 60 micron is produced by star-formation and unrelated to the presence of an obscuring torus. We also find that the toroid cannot be flat, but is rather conical or flaring. The density is most likely constant with radius, and the size is relatively large with an inner radius around 10 pc. A direct comparison with radio galaxy Cygnus A yields a best fit for a conical disk with constant surface density, and a size from 10 to 30 pc assuming the far-infrared emission is due to star-formation in the host galaxy.

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arXiv2002

Understanding the spectra of isolated Herbig stars in the frame of a passive disk model

We discuss spectral energy distributions of a sample of Herbig Ae/Be stars in the context of a passive irradiated disk model. The data have been presented earlier by Meeus et al (2001), and preliminary interpretations of these data were given in that paper. While the spectra of Herbig Ae stars all show similarities, there is significant variation between the spectra, in particular in the shape of the mid-IR rise and in the presence or absence of a silicate feature. We explore the hypothesis that all these different spectra can be interpreted as pure disk spectra without additional components. Using the model of Dullemond, Dominik and Natta (2001) we deduce the disk parameters of a number of the sources, and find that for a large fraction of investigated sources, satisfactory fits can be obtained. The derived model parameters show that some group Ia sources can only be fit with radially increasing surface densities, indicating the presence of depleted inner disk regions. The steep-sloped SEDs of group IIa sources can be fit with very compact disks, probably representing disks with collapsed outer regions. The largest difficulties arise from sources that do not show significant silicate emission features. Our attempts to explain these objects with a pure geometric effect are only partially successful. It seems that these stars indeed require a strong depletion of small silicate grains.

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