arXiv subjects

J. E. Bjorkman

Publications and source records attributed to J. E. Bjorkman.

At least 37 records · Page 2Linked to original sources

arXiv2007

Achernar: Rapid Polarization Variability as Evidence of Photospheric and Circumstellar Activity

We present the results of a high accuracy ($σ\approx 0.005%$) polarization monitoring of the Be Star Achernar that was carried out between July 7th and November 5th, 2006. Our results indicate that, after a near quiescent phase from 1998 to 2002, Achernar is presently in an active phase and has built a circumstellar disk. We detect variations both in the polarization level and position angle in timescales as short as one hour and as long as several weeks. Detailed modeling of the observed polarization strongly suggests that the short-term variations originate from discrete mass ejection events which produce transient inhomogeneities in the inner disk. Long-term variations, on the other hand, can be explained by the formation of an inner ring following one or several mass ejection events.

astro-ph↗

arXiv2006

CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks

We present the first K'-band, long-baseline interferometric observations of the northern Be stars gamma Cas, phi Per, zeta Tau, and kappa Dra. The measurements were made with multiple telescope pairs of the CHARA Array interferometer, and in every case the observations indicate that the circumstellar disks of the targets are resolved. We fit the interferometric visibilities with predictions from a simple disk model that assumes an isothermal gas in Keplerian rotation. We derive fits of the four model parameters (disk base density, radial density exponent, disk normal inclination, and position angle) for each of the targets. The resulting densities are in broad agreement with prior studies of the IR excess flux and the resulting orientations generally agree with those from interferometric H-alpha and continuum polarimetric observations. We find that the angular size of the K' disk emission is smaller than that determined for the H-alpha emission, and we argue that the difference is the result of a larger H-alpha opacity and the relatively larger neutral hydrogen fraction with increasing disk radius. All the targets are known binaries with faint companions, and we find that companions appear to influence the interferometric visibilities in the cases of phi Per and kappa Dra. We also present contemporaneous observations of the H-alpha, H-gamma, and Br-gamma emission lines. Synthetic model profiles of these lines that are based on the same disk inclination and radial density exponent as derived from the CHARA Array observations match the observed emission line strength if the disk base density is reduced by approximately 1.7 dex.

astro-ph↗

arXiv2006

Properties of the $δ$ Scorpii Circumstellar Disk from Continuum Modeling

We present optical $WBVR$ and infrared $JHKL$ photometric observations of the Be binary system $δ$ Sco, obtained in 2000--2005, mid-infrared (10 and $18 μ$m) photometry and optical ($λλ$ 3200--10500 Å) spectropolarimetry obtained in 2001. Our optical photometry confirms the results of much more frequent visual monitoring of $δ$ Sco. In 2005, we detected a significant decrease in the object's brightness, both in optical and near-infrared brightness, which is associated with a continuous rise in the hydrogen line strenghts. We discuss possible causes for this phenomenon, which is difficult to explain in view of current models of Be star disks. The 2001 spectral energy distribution and polarization are succesfully modeled with a three-dimensional non-LTE Monte Carlo code which produces a self-consistent determination of the hydrogen level populations, electron temperature, and gas density for hot star disks. Our disk model is hydrostatically supported in the vertical direction and radially controlled by viscosity. Such a disk model has, essentially, only two free parameters, viz., the equatorial mass loss rate and the disk outer radius. We find that the primary companion is surrounded by a small (7 $R_\star$), geometrically-thin disk, which is highly non-isothermal and fully ionized. Our model requires an average equatorial mass loss rate of $1.5\times 10^{-9} M_{\sun}$ yr$^{-1}$.

astro-ph↗

arXiv2006

Modeling the Structure of Hot Star Disks: a Critical Evaluation of the Viscous Decretion Scenario

We present self-consistent solutions for the disk structure of classical Be stars. Our disk model is hydrostatically supported in the vertical direction and the radial structure is governed by viscosity ($α$-disks). We perform three-dimensional non-LTE Monte Carlo simulations to calculate simultaneously both the equilibrium temperature and Hydrogen level populations and to solve self-consistently for the density structure of the disk. We discuss the general properties of the solution for the disk structure and test our model against observations of $δ$ Scorpii. Our results confirm that a viscous decretion disk model is consistent with these observations.

astro-ph↗

arXiv2005

NLTE Monte Carlo Radiative Transfer: I. The Thermal Properties of Keplerian Disks around Classical Be Stars

We present a 3-D NLTE Monte Carlo radiative transfer code that we use to study the temperature and ionization structure of Keplerian disks around Classical Be stars. The method we employ is largely similiar to the Monte Carlo transition probability method developed by Lucy. Here we present a simplification of his method that avoids the use of the macro atom concept. Our investigations of the temperature structure of Be star disks show that the disk temperature behavior is a hybrid between the behavior of Young Stellar Object (YSO) disks and Hot Star winds. The optically thick inner parts of Be star disks have temperatures that are similar to YSO disks, while the optically thin outer parts are like stellar winds. Thus, the temperature at the disk midplane initially drops, reaching a minimum at 3--5 stellar radii, after which it rises back to the optically thin radiative equilibrium temperature at large distances. On the other hand, the optically thin upper layers of the disk are approximately isothermal -- a behavior that is analogous to the hot upper layers of YSO disks. We also find that the disks are fully ionized, as expected, but there is an ionization minimum in the vicinity of the temperature minimum. Finally, we find that, despite the complex temperature structure, the infrared excess is well-approximated by an equivalent isothermal disk model whose temperature is about 60% of the stellar temperature. This is largely because, at long wavelengths, the effective photosphere of the disk is located in its isothermal regions.

astro-ph↗

arXiv2005

The polarisation signature from microlensing of circumstellar envelopes in caustic crossing events

The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarisation signal -- arising from differential magnification across the otherwise symmetric source. In a previous paper, polarimetric variations from point lensing of a circumstellar envelope were considered, as would be suitable for an extended envelope around a red giant. In this work we examine the polarisation in the context of caustic crossing events, the scenario which represents the most easily accessible situation for actually observing a polarisation signal in Galactic microlensing.

astro-ph↗

arXiv2005

Observational Properties of Proto-planetary Disk Gaps

We study the effects of an annular gap induced by an embedded proto-planet on disk scattered light images and the infrared spectral energy distribution. We find that the outer edge of a gap is brighter in the scattered light images than a similar location in a gap-free disk. The stellar radiation that would have been scattered by material within in the gap is instead scattered by the disk wall at the outer edge of the gap, producing a bright ring surrounding the dark gap in the images. Given sufficient resolution, such gaps can be detected by the presence of this bright ring in scattered light images. A gap in a disk also changes the shape of the SED. Radiation that would have been absorbed by material in the gap is instead reprocessed by the outer gap wall. This leads to a decrease in the SED at wavelengths corresponding to the temperature at the radius of the missing gap material and a corresponding flux increase at longer wavelengths corresponding to the temperature of the outer wall. We note that, unlike an inner hole in the disk, the presence of an annular gap does not change the bolometric IR flux; it simply redistributes the radiation, previously produced by material within the gap, to longer wavelengths. This implies that the changes in the SED generally will be smaller for gaps than holes. Although it will be difficult on the basis of the SED alone to distinguish between the presence of a gap and other physical effects, the level of changes can be sufficiently large to be measurable with current instruments (e.g., Spitzer).

astro-ph↗

arXiv2005

2-D and 3-D Radiation Transfer Models of High-Mass Star Formation

2-D and 3-D radiation transfer models of forming stars generally produce bluer 1-10 micron colors than 1-D models of the same evolutionary state and envelope mass. Therefore, 1-D models of the shortwave radiation will generally estimate a lower envelope mass and later evolutionary state than multidimensional models. 1-D models are probably reasonable for very young sources, or longwave analysis (wavelengths > 100 microns). In our 3-D models of high-mass stars in clumpy molecular clouds, we find no correlation between the depth of the 10 micron silicate feature and the longwave (> 100 micron) SED (which sets the envelope mass), even when the average optical extinction of the envelope is >100 magnitudes. This is in agreement with the observations of Faison et al. (1998) of several UltraCompact HII (UCHII) regions, suggesting that many of these sources are more evolved than embedded protostars. We have calculated a large grid of 2-D models and find substantial overlap between different evolutionary states in the mid-IR color-color diagrams. We have developed a model fitter to work in conjunction with the grid to analyze large datasets. This grid and fitter will be expanded and tested in 2005 and released to the public in 2006.

astro-ph↗

arXiv2005

Modeling the Photopolarimetric Variability of AA Tau

We present Monte Carlo scattered light models of a warped disk that reproduce the observed photopolarimetric variability of the classical T Tauri star AA Tau. For a system inclination of 75 deg and using an analytic description for a warped inner disk, we find that the shape and amplitude of the photopolarimetric variability are reproduced with a warp which occults the star, located at 0.07 au, amplitude 0.016 au, extending over radial and azimuthal ranges 0.0084 au and 145 deg. We also show a time sequence of high spatial resolution scattered light images, showing a dark shadow cast by the warp sweeping round the disk. Using a modified smooth particle hydrodynamics code, we find that a stellar dipole magnetic field of strength 5.2 kG, inclined at 30 deg to the stellar rotation axis can reproduce the required disk warping to explain AA Tau's photopolarimetric variability.

astro-ph↗

arXiv2004

Observations and modeling of the inner disk region of T Tauri stars

We present observations of four T Tauri stars using long baseline infrared interferometry from the Palomar Testbed Interferometer. The target sources, T Tau N, SU Aur, RY Tau and DR Tau, are all known to be surrounded by dusty circumstellar disks. The observations directly trace the inner regions (< 1 AU) of the disk and can be used to constrain the physical properties of this material. For three of the sources observed, the infrared emission is clearly resolved. We first use geometric models to characterize the emission region size, which ranges from 0.04 to 0.3 AU in radius. We then use Monte Carlo radiation transfer models of accretion disks to jointly model the spectral energy distribution and the interferometric observations with disk models including accretion and scattering. With these models, we are able to reproduce the data set with extended emission arising from structures larger than 10 milliarcseconds contributing less than 6% of the K band emission, consistent with there being little or no envelope remaining for these Class II sources (d log(lambda*F_lambda)/d log(lambda) ~ -2 to 0 in the infrared). The radiation transfer models have inner radii for the dust similar to the geometric models; however, for RY Tau emission from gas within the inner dust radius contributes significantly to the model flux and visibility at infrared wavelengths. The main conclusion of our modeling is that emission from inner gas disks (between the magnetic truncation radius and the dust destruction radius) can be a significant component in the inner disk flux for sources with large inner dust radii.

astro-ph↗

arXiv2004

2-D Models of Protostars: III. Effects of Stellar Temperature

We model how the mid-infrared colors of Young Stellar Objects (YSOs) vary with stellar temperature. The spectral energy distribution (SED) of each object has contributions from thermal emission of circumstellar dust, from direct stellar photospheric emission, and from scattered stellar emission. We first isolate the effects of stellar contributions (direct+scattered) to the SED using homologous "Class I" models, where the shape of the thermal contribution to the SED remains constant. Stellar light contributes relatively more to the MIR emission of YSOs with lower temperature stars (Tstar ~4000 K) because the emission peak wavelength of the star is closer to that of the thermal radiation. Next we consider realistic Class 0, I, and II source models and find that the other dominant effect of varying stellar temperature on YSO SEDs is that of the inner disk wall: In high-Tstar models, the dust destruction radius is much further out with a consequently larger inner disk wall that contributes relatively more to the 2-10 um flux. This effect partially offsets that of the stellar contribution leading to varying behaviors of the 2-10 um flux. Our modeled MIR protostellar colors have implications for interpretations of Spitzer IRAC observations of star formation regions: It is commonly assumed that the slope of the SED at 1-10 um is directly related to evolutionary state. We show that inclination effects, aperture size, scattered light, and stellar temperature cause a broad spread in the colors of a source at a single evolutionary state. Color magnitude diagrams can help sort out these effects by separating sources with different Tstar based on their different brightness (for sources at the same distance).

astro-ph↗

arXiv2004

Modeling the Circumstellar Disk of $ζ$ Tauri

We present a model for the disk of the classical Be star $ζ$ Tauri. The model consists of a Keplerian rotating disk with a power-law surface density and a vertical density distribution that follows from the balance between the thermal gas pressure and the z-component of the stellar gravitation. The opening angle of such a disk is not a fixed value but increases with the distance to the star (flared disk). We use a Monte Carlo code that solves simultaneously the thermal equilibrium, the statistical equilibrium and the hydrostatic equilibrium equations. The result is a self-consistent solution for the electron temperature, level populations and gas density distribution. It follows from this model that the only free parameter is the disk density. All other parameters (e.g. shape of the disk surface) are self-consistently calculated given the disk density. In this paper we present a successful fit for $ζ$ Tauri data using this model.

astro-ph↗

arXiv2004

The Structure of Brown Dwarf Circumstellar Disks

We present synthetic spectra for circumstellar disks that are heated by radiation from a central brown dwarf. Under the assumption of vertical hydrostatic equilibrium, our models yield scaleheights for brown dwarf disks in excess of three times those derived for classical T Tauri (CTTS) disks. If the near-IR excess emission observed from brown dwarfs is indeed due to circumstellar disks, then the large scaleheights we find could have a significant impact on the optical and near-IR detectability of such systems. Our radiation transfer calculations show that such highly flared disks around brown dwarfs will result in a large fraction of obscured sources due to extinction of direct starlight by the disk over a wide range of sightlines. The obscured fraction for a 'typical' CTTS is less than 20%. We show that the obscured fraction for brown dwarfs may be double that for CTTS, but this depends on stellar and disk mass. We also comment on possible confusion in identifying brown dwarfs via color-magnitude diagrams: edge-on CTTS display similar colors and magnitudes as a face-on brown dwarf plus disk systems.

astro-ph↗

arXiv2003

Effects of Grain Size on the Spectral Energy Distribution of Dusty Circumstellar Envelopes

We study the effects of dust grain size on the spectral energy distribution (SED) of spherical circumstellar envelopes. Based on the self-similarity relations of dusty SEDs derived by Ivezic & Elitzur (1997), we expect an approximate invariance of the IR SED for models with different grain sizes. Approximate invariance follows from the fact that differently sized grains have similar optical properties at long wavelengths where the dust reprocesses the starlight. In this paper, we discuss what are the physical requirements on the model parameters to maintain the approximate invariance of the IR SED. Single grain size models are studied for a wide range of grain sizes in three optical depth regimes. In this study, we find limits for the cases where the IR SED is and is not capable of conveying information about grain sizes, and to what extent it does so. We find that approximate invariance occurs for a much larger range of grain sizes than previously believed, and, when approximate invariance holds, the SED is controlled mainly by one parameter, the reprocessing optical depth, a quantity that measures the fraction of starlight that is absorbed by the dust grains. Models with a grain size distribution are studied as well. For these models, we find that, in many instances, the concept of approximate invariance may be extended from the IR SED to all wavelengths. This means that, for a wide range of optical depths, models with different grain size distributions will produce very similar SEDs and, hence, the reprocessing optical depth is the only quantity that can be unambiguously obtained from the SED. The observational consequences of this result are discussed in detail.

astro-ph↗

arXiv2003

2-D Radiative Transfer in Protostellar Envelopes: II. An Evolutionary Sequence

We present model spectral energy distributions, colors, polarization, and images for an evolutionary sequence of a low-mass protostar from the early collapse stage (Class 0) to the remnant disk stage (Class III). We find a substantial overlap in colors and SEDs between protostars embedded in envelopes (Class 0-I) and T Tauri disks (Class II), especially at mid-IR wavelengths. Edge-on Class I-II sources show double-peaked spectral energy distributions, with a short-wavelength hump due to scattered light and the long-wavelength hump due to thermal emission. These are the bluest sources in mid-IR color-color diagrams. Since Class 0 and I sources are diffuse, the size of the aperture over which fluxes are integrated has a substantial effect on the computed colors, with larger aperture results showing significantly bluer colors. This causes overlap in color-color diagrams between all evolutionary states, especially in the mid-IR. However the near-IR polarization of the Class 0 sources is much higher than the Class I-II sources, providing a means to separate these evolutionary states. We varied the grain properties in the circumstellar envelope, allowing for larger grains in the disk midplane and smaller in the envelope. We find that grain growth in disks of Class I sources can be detected at wavelengths greater than 100 $μ$m. Our image calculations predict that the diffuse emission from edge-on Class I and II sources should be detectable in the mid-IR with the Space Infrared Telescope Facility (SIRTF) in nearby star forming regions (out to several hundred parsecs).

astro-ph↗

arXiv2003

2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources

We present 2-D radiation transfer models of Class I Protostars and show the effect of including more realistic geometries on the resulting spectral energy distributions and images. We begin with a rotationally flattened infalling envelope as our comparison model, and add a flared disk and bipolar cavity. The disk affects the spectral energy distribution most strongly at edge-on inclinations, causing a broad dip at about 10 um (independent of the silicate feature) due to high extinction and low scattering albedo in this wavelength region. The bipolar cavities allow more direct stellar+disk radiation to emerge into polar directions, and more scattering radiation to emerge into all directions. The wavelength-integrated flux, often interpreted as luminosity, varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much flux as edge-on, depending on geometry. Thus, observational estimates of luminosity should take into account the inclination of a source. The envelopes with cavities are significantly bluer in near-IR and mid-IR color-color plots than those without cavities. Using 1-D models to interpret Class I sources with bipolar cavities would lead to an underestimate of envelope mass and an overestimate of the implied evolutionary state. We compute images at near-, mid-, and far-IR wavelengths. We find that the mid-IR colors and images are sensitive to scattering albedo, and that the flared disk shadows the midplane on large size scales at all wavelengths plotted. Finally, our models produce polarization spectra which can be used to diagnose dust properties, such as albedo variations due to grain growth. Our results of polarization across the 3.1 um ice feature agree well with observations for ice mantles covering 5% of the radius of the grains.

astro-ph↗

arXiv2003

Constraints on a planetary origin for the gap in GM Aurigae's protoplanetary disc

The unusual spectral energy distribution (SED) of the classical T Tauri star GM Aurigae provides evidence for the presence of an inner disc hole extending to several au. Using a combination of hydrodynamical simulations and Monte Carlo radiative transport, we investigate whether the observed SED is consistent with the inner hole being created and maintained by an orbiting planet. We show that an ~ 2 M_Jupiter planet, orbiting at 2.5 au in a disc with mass 0.047 M_sun and radius 300 au, provides a good match both to the SED and to CO observations which constrain the velocity field in the disc. A range of planet masses is allowed by current data, but could in principle be distinguished with further observations between 3 and ~ 20 microns. Future high precision astrometric instruments should also be able to detect the motion of the central star due to an orbiting Jupiter mass planet. We argue that the small number of T Tauri stars with SEDs resembling that of GM Aur is broadly consistent with the expected statistics of embedded migrating planets.

astro-ph↗

arXiv2002

Spatial study with the VLT of a new resolved edge-on circumstellar dust disk discovered at the periphery of the rho Ophiuchi dark cloud

We report the discovery in NIR with SofI at the NTT of a resolved circumstellar dust disk around a 2MASS source at the periphery of the rho Ophiuchi dark cloud. We present follow-up observations in J, H, and Ks-band obtained with ISAAC at the VLT, under 0.4"-seeing conditions, which unveil a dark dust lane oriented East-West between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 2.15" (300 AU at 140 pc), and a width of 1.2" (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (Av=2.1\pm2.6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called ``Pixel NIR Color Mapping'', which allows to visualize directly the NIR colors of the nebula pixels. Thanks to this method we identify a ridge, 0.3" (40 AU at 140 pc) to the north of the dark lane and parallel to it, which displays a NIR color excess. This ridge corresponds to an unusual increase of brightness from J to Ks, which is also visible in the NTT observation obtained 130 days before the VLT one. We also find that the northern nebula shows ~3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i=86\pm1 deg, correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar asymmetrical NIR color properties of this object.

astro-ph↗