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)Comment: 28 pages, 8 figures, accepted for publication in Ap

The Onset of Planet Formation in Brown Dwarf Disks

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub-micron-sized dust grains accompanied by dust settling toward the disk mid-plane. Here we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.Comment: Published in Science 2005, vol 310, 834; 3 pages in final format, 4 figures + 8 pages Supporting Online Material. For final typeset, see http://www.sciencemag.org/cgi/content/abstract/310/5749/834?eto

Trapping dust particles in the outer regions of protoplanetary disks

Aims. We attempt to explain grain growth to mm sized particles and their retention in the outer regions of protoplanetary disks, as observed at sub-mm and mm wavelengths, by investigating whether strong inhomogeneities in the gas density profiles can decelerate excessive radial drift and help the dust particles to grow. Methods. We use coagulation/fragmentation and disk-structure models, to simulate the evolution of dust in a bumpy surface density profile, which we mimic with a sinusoidal disturbance. For different values of the amplitude and length scale of the bumps, we investigate the ability of this model to produce and retain large particles on million-year timescales. In addition, we compare the pressure inhomogeneities considered in this work with the pressure profiles that come from magnetorotational instability. Using the Common Astronomy Software Applications ALMA simulator, we study whether there are observational signatures of these pressure inhomogeneities that can be seen with ALMA. Results. We present the conditions required to trap dust particles and the corresponding calculations predicting the spectral slope in the mm-wavelength range, to compare with current observations. Finally, we present simulated images using different antenna configurations of ALMA at different frequencies, to show that the ring structures will be detectable at the distances of either the Taurus Auriga or Ophiucus star-forming regions

KH15D: a star eclipsed by a large scale dusty vortex?

We propose that the large photometric variations of KH15D are due to an eclipsing swarm of solid particles trapped in giant gaseous vortex rotating at \~0.2 AU from the star. The efficiency of the capture-in-vortex mechanism easily explains the observed large optical depth. The weaker opacity at mid-eclipse is consistent with a size segregation of the particles toward the center of the vortex. This dusty structure must extend over ~1/3 of an orbit to account for the long eclipse duration. The estimated size of the trapped particles is found to range from 1 to 10cm, consistent with the gray extinction of the star. The observations of KH15D support the idea that giant vortices can grow in circumstellar disks and play a central role in planet formation.Comment: Accepted in ApJ Letters - 4 pages - 2 figure

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.Comment: Accepted for publication in Astronomy & Astrophysic

Spectral line profiles changed by dust scattering in heavily obscured young stellar objects

It is known that scattering of radiation by circumstellar dust can strongly change the line profiles in stellar spectra. This hampers the analysis of spectral lines originating in the emitting regions of heavily obscured young stars. To calculate the line profile of the scattered radiation, we suggest to use the approximation of remote scattering particles. This approximation assumes that the scattering dust grains are at a distance from the star that is much larger than the characteristic size of the emitting region. Using this method, we calculated the line profiles of several simple models. They show the H alpha line profiles of Herbig AeBe stars in the presence and absence of motionless or moving dust

A Keplerian gaseous disk around the B0 star R Mon

We present high-angular resolution observations of the circumstellar disk around the massive Herbig Be star R Mon (M~8 Msun) in the continuum at 2.7mm and 1.3mm and the CO 1->0 and 2->1 rotational lines. Based on the new 1.3mm continuum image we estimate a disk mass (gas+dust) of 0.007 Msun and an outer radius of <150 AU. Our CO images are consistent with the existence of a Keplerian rotating gaseous disk around this star. Up to our knowledge, this is the most clear evidence for the existence of Keplerian disks around massive stars reported thus far. The mass and physical characteristics of this disk are similar to thoseof the more evolved T Tauri stars and indicate a shorter timescale for the evolution and dispersal of circumstellar disks around massive stars which lose most of their mass before the star becomes visible.Comment: 5 page

Association of Retinal Vascular Caliber and Age-Related Macular Degeneration in Patients With the Acquired Immunodeficiency Syndrome.

PurposeTo evaluate the relationship between retinal vascular caliber and AMD in patients with AIDS.MethodsParticipants enrolled in the Longitudinal Study of the Ocular Complications of AIDS had retinal photographs taken at enrollment. Retinal vascular caliber (central retinal artery equivalent [CRAE] and central retinal vein equivalent [CRVE]) and intermediate-stage AMD were determined from these retinal photographs. Photographs were evaluated by graders at a centralized reading center, using the Age-Related Eye Disease Study grading system for AMD and semiautomated techniques for evaluating retinal vascular caliber.ResultsOf the 1171 participants evaluated, 110 (9.4%) had AMD and 1061 (90.6%) did not. Compared with participants without AMD, participants with AMD had larger mean CRAEs (151 ± 16 μm versus 147 ± 16 μm; P = 0.009) and mean CRVEs (228 ± 24 μm versus 223 ± 25 μm; P = 0.02). The unadjusted differences were: CRAE, 4.3 μm (95% confidence interval [CI] 1.1-7.5; P = 0.009) and CRVE, 5.5 μm (95% CI 0.7-10.3; P = 0.02). After adjustment for age, race/ethnicity, sex, human immunodeficiency syndrome (HIV) transmission category, smoking, enrollment and nadir CD4+ T cells, and enrollment and maximum HIV load, the differences between patients with and without AMD were as follows: CRAE, 5.4 μm (95% CI 2.3-8.5; P = 0.001) and CRVE, 6.0 μm (95% CI 1.4-10.6; P = 0.01).ConclusionsIn patients with AIDS, AMD is associated with greater retinal arteriolar and venular calibers, suggesting a role for shared pathogenic mechanisms, such as persistent systemic inflammation

Vertical structure models of T Tauri and Herbig Ae/Be disks

In this paper we present detailed models of the vertical structure (temperature and density) of passive irradiated circumstellar disks around T Tauri and Herbig Ae/Be stars. In contrast to earlier work, we use full frequency- and angle-dependent radiative transfer instead of the usual moment equations. We find that this improvement of the radiative transfer has strong influence on the resulting vertical structure of the disk, with differences in temperature as large as 70 %. However, the spectral energy distribution (SED) is only mildly affected by this change. In fact, the SED compares reasonably well with that of improved versions of the Chiang & Goldreich (CG) model. This shows that the latter is a reasonable model for the SED, in spite of its simplicity. It also shows that from the SED alone, little can be learned about the vertical structure of a passive circumstellar disk. The molecular line emission from these disks is more sensitive to the vertical temperature and density structure, and we show as an example how the intensity and profiles of various CO lines depend on the adopted disk model. The models presented in this paper can also serve as the basis of theoretical studies of e.g. dust coagulation and settling in disks.Comment: 12 pages, 15 figures, accepted for publication in A&