arXiv subjects

J. E. Bjorkman

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

40 records · Page 3Linked to original sources

arXiv2001

Infrared Signatures of Protoplanetary Disk Evolution

We investigate the observational signatures of a straightforward evolutionary scenario for protoplanetary disks in which the disk mass of small (50 micron) particles decreases homologously with time, but the disk structure and stellar parameters do not change. Our goal is to identify optimal infrared spectral indicators of the existence of disks, their structure, and mass evolution that may be tested with the upcoming SIRTF mission. We present simulated spectral energy distributions and colors over a wide range of masses. The SED is most sensitive to disk mass in the far-IR and longer wavelengths, which is already known from millimeter and radio observations. As the disk mass decreases, the excess emission of the disk over the stellar photosphere diminishes more rapidly at the longest rather than at short wavelengths. At near-infrared wavelengths, the disk remains optically thick to stellar radiation over a wide range of disk mass, resulting in a slower decline in the SED in this spectral regime. Therefore, near-IR excesses (K-L) provide a robust means of detecting disks in star clusters down to 1E-7 solar masses, while the far-IR excess probes the disk mass. Reducing the disk mass results in a clear progression in color-color diagrams with low mass disks displaying the bluest colors. We interpret color-color diagrams for Taurus-Auriga sources in the context of decreasing disk mass.

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arXiv2001

The Spectral Energy Distribution of HH30 IRS: Constraining The Circumstellar Dust Size Distribution

We present spectral energy distribution (SED) models for the edge-on classical T Tauri star HH30 IRS that indicate dust grains have grown to larger than 50 microns within its circumstellar disk. The disk geometry and inclination are known from previous modeling of multiwavelength Hubble Space Telescope images and we use the SED to constrain the dust size distribution. Model spectra are shown for different circumstellar dust models: a standard ISM mixture and larger grain models. As compared to ISM grains, the larger dust grain models have a shallower wavelength dependent opacity. Models with the larger dust grains provide a good match to the currently available data, but mid and far-IR observations are required to more tightly constrain the dust size distribution. The accretion luminosity in our models is L_acc<0.2 L_star corresponding to an accretion rate of 4E-9M_sun/yr. Dust size distributions that are simple power-law extensions (i.e., no exponential cutoff) yield acceptable fits to the optical/near-IR but too much emission at mm wavelengths and require larger disk masses. Such a simple size distribution would not be expected in an environment such as the disk of HH30 IRS, particularly over such a large range in grain sizes. However, its ability to adequately characterize the grain populations may be determined from more complete observational sampling of the SED in the mid to far-IR.

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arXiv2001

Radiative Equilibrium and Temperature Correction in Monte Carlo Radiation Transfer

We describe a general radiative equilibrium and temperature correction procedure for use in Monte Carlo radiation transfer codes with sources of temperature-independent opacity, such as astrophysical dust. The technique utilizes the fact that Monte Carlo simulations track individual photon packets, so we may easily determine where their energy is absorbed. When a packet is absorbed, it heats a particular cell within the envelope, raising its temperature. To enforce radiative equilibrium, the absorbed packet is immediately re-emitted. To correct the cell temperature, the frequency of the re-emitted packet is chosen so that it corrects the temperature of the spectrum previously emitted by the cell. The re-emitted packet then continues being scattered, absorbed, and re-emitted until it finally escapes from the envelope. As the simulation runs, the envelope heats up, and the emergent spectral energy distribution (SED) relaxes to its equilibrium value, without iteration. This implies that the equilibrium temperature calculation requires no more computation time than the SED calculation of an equivalent pure scattering model with fixed temperature. In addition to avoiding iteration, our method conserves energy exactly, because all injected photon packets eventually escape. Furthermore, individual packets transport energy across the entire system because they are never destroyed. This long-range communication, coupled with the lack of iteration, implies that our method does not suffer the convergence problems commonly associated with lambda-iteration. To verify our temperature correction procedure, we compare our results to standard benchmark tests, and finally we present the results of simulations for two-dimensional axisymmetric density structures.

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arXiv1998

Supernova 1987A: Rotation and a Binary Companion

In this paper we provide a possible link between the structure of the bipolar nebula surrounding SN1987A and the properties of its progenitor star. A Wind Blwon Bubble (WBB) scenario is emplyed, in which a fast, tenuous wind from a Blue Supergiant expands into a slow, dense wind, expelled during an earlier Red Supergiant phase. The bipolar shapre develops due to a pole-to-equator density contrast in the slow wind (ie, the slow wind forms a slow torus). We use the Wind Compressed Disk (WCD) model of Bjorkman & Cassinelli (1992) to determine the shape of the slow torus. In the WCD scenario, the shape of the torus is determined by the rotation of the progenitor star. We then use a self-similar semi-analytical method for wind blown bubble evolution to determine the shape of the resulting bipolar nebula. We find that the union of the wind-compressed-disk and bipolar-wind-blown- bubble models allows us to recover the salient properties of SN1987A's circumstellar nebula. In particular, the size, speed and density of SN1987A's inner ring are easily reproduced in our calculations. An exploration of parameter space shows the the red supergiant progenitor must be been rotating at > 0.3 of its breakup speed. We conclude that the progenitor was most likely spun up by a merger with a binary companion. Using a simple model for the binary merger we find that the companion is likely to have had a mass > 0.5 M_sun.

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