11 research outputs found
On the formation of massive stars
We calculate numerically the collapse of slowly rotating, non-magnetic,
massive molecular clumps, which conceivably could lead to the formation of
massive stars. Because radiative acceleration on dust grains plays a critical
role in the clump's dynamical evolution, we utilize a wavelength-dependent
radiation transfer and a three component dust model: amorphous carbon
particles, silicates and "dirty ice"-coated silicates. We do not spatially
resolve the innermost regions of the molecular clump and assume that all
material in the innermost grid cell accretes onto a single object. We introduce
a semi-analytical scheme for augmenting existing evolution tracks of pre-main
sequence protostars by including the effects of accretion. By considering an
open outermost boundary, an arbitrary amount of material could, in principal,
be accreted onto this central star. However, for the three cases considered
(30, 60, and 120 solar masses originally within the computation grid),
radiation acceleration limited the final masses to 31.6, 33.6, and 42.9 solar
masses, respectively, for wavelength-dependent radiation transfer and to 19.1,
20.1, and 22.9 solar masses for comparison simulations with grey radiation
transfer. We demonstrate that massive stars can in principle be formed via
accretion through a disk. We conclude with the warning that a careful treatment
of radiation transfer is a mandatory requirement for realistic simulations of
the formation of massive stars.Comment: 39 pages, 13 figures, 4 tables, AASTEX v5.0, accepted by Ap
HST/NICMOS Imaging of Disks and Envelopes Around Very Young Stars
We present HST/NICMOS observations with 0.1" (15 AU) resolution of six young
stellar objects in the Taurus star-formation region. The targets of our survey
are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS
04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu
Tau/1) associated with Herbig Haro jets. The broad-band images show that the
near-infrared radiation from these sources is dominated by light scattered from
dusty circumstellar material distributed in a region 10 - 15 times the size of
our solar system. Although the detailed morphologies of the individual objects
are unique, the observed young stellar objects share common features. All of
the circumstellar reflection nebulae are crossed by dark lanes from 500 - 900
AU in extent and from less than 50 to 350 AU in apparent thickness. The
absorption lanes extend perpendicular to known optical and millimeter outflows
in these sources. We interpret the dark lanes as optically thick circumstellar
disks seen in silhouette against bright reflection nebulosity. The bipolar
reflection nebulae extending perpendicular to the dust lanes appear to be
produced by scattering from the upper and lower surfaces of the disks and from
dusty material within or on the walls of the outflow cavities. Out of five
objects in which the central source is directly detected, two are found to be
subarcsecond binaries. This mini-survey is the highest resolution near-infrared
study to date of circumstellar environments around solar-type stars with age <=
1 Myr.Comment: 34 pages, 4 figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/topics/disks/disks.html ;
accepted for publication in AJ (March 1999 issue
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.Comment: ApJ Accepte
On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga
[abridged] We present high resolution optical spectra of stars in
Taurus-Auriga whose circumstellar environment suggests that they are less
evolved than optically revealed T Tauri stars. Many of the stars are seen only
via scattered light. These spectra are used to search for differences between
stars which power Herbig-Haro flows and stars which do not, and to reassess the
evolutionary state of so-called protostars (Class I stars) relative to
optically revealed T Tauri stars (Class II stars). The stellar mass
distribution of Class I stars is similar to that of Class II stars and includes
3 Class I brown dwarfs. Class I stars in Taurus are slowly rotating; the
angular momentum of a young star appears to dissipate prior to the optically
revealed T Tauri phase. The mass accretion rates of Class I stars are
surprisingly indistinguishable from those of Class II stars; they do not have
accretion dominated luminosities. We confirm previous results that find larger
forbidden-line emission associated with Class I stars than Class II stars. We
suggest that this is caused by an orientation bias that allows a more direct
view of the somewhat extended forbidden emission line regions than the obscured
stellar photospheres, rather than because of larger mass outflow rates.
Overall, the similar masses, luminosities, rotation rates, mass accretion
rates, mass outflow rates, and millimeter flux densities of Class I and Class
II stars are best explained by a scenario in which most Class I stars are no
longer in the main accretion phase and are older than traditionally assumed.
Similarly, although stars which power Herbig-Haro flows appear to have larger
mass outflow rates, their stellar and circumstellar properties are generally
indistinguishable from those of stars that do not power these flows.Comment: 84 pages, including 21 figures; accepted for publication in Ap
Efficient radiative transfer in dust grain mixtures
The influence of a dust grain mixture consisting of spherical dust grains
with different radii and/or chemical composition on the resulting temperature
structure and spectral energy distribution of a circumstellar shell is
investigated. The comparison with the results based on an approximation of dust
grain parameters representing the mean optical properties of the corresponding
dust grain mixture reveal that (1) the temperature dispersion of a real dust
grain mixture decreases substantially with increasing optical depth, converging
towards the temperature distribution resulting from the approximation of mean
dust grain parameters, and (2) the resulting spectral energy distributions do
not differ by more than 10% if >= 2^5 grain sizes are considered which
justifies the mean parameter approximation and the many results obtained under
its assumption so far. Nevertheless, the dust grain temperature dispersion at
the inner boundary of a dust shell may amount to >>100K and has therefore to be
considered in the correct simulation of, e.g., chemical networks.
In order to study the additional influence of geometrical effects, a
two-dimensional configuration -- the HH30 circumstellar disk -- was considered,
using model parameters from Cotera et al. (2001) and Wood et al. (2002). A
drastic inversion of the large to small grain temperature distribution was
found within the inner approx. 1AU of the disk.Comment: ApJ, accepte
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.Comment: 24 pages, 8 figure
Optical and Near-Infrared Model Images of the Circumstellar Environments of Classical T Tauri Stars
We describe model calculations of optical and near-infrared scattered light images expected from class II T Tauri stars-the star-plus-disk systems. The parameters controlling the disk shape, size, and mass are chosen to be within theoretically and observationally derived limits. We restrict our models to nearly edge-on disks, since for lower inclinations the central starlight is many orders of magnitude greater than the radiation scattered in the disk. In addition to model flux images, we calculate spectral energy distributions for pole-on viewing using approximations for hat and flared disks. We find that direct imaging of edge-on disks can provide only estimates of the scale height at large distances from the central star and an estimate of the disk mass. The images are rather insensitive to the inner disk radius and the degree of Baring, provided the scale height is axed at large radii. Spectral energy distribution modeling is required to constrain the inner disk radius and the degree of flaring.We apply our models to recent Hubble Space Telescope (HST) images of HH 30 IRS and investigate whether the scattered light images could have been produced by starlight scattering off the walls of jet-carved cavities in infalling envelopes associated with the embedded class I sources. We find that while the class I infalling envelope plus cavity model qualitatively resembles the HST images, the spatial extent of the model images is too large. Edge-on disk models appear to provide better fits to the data and enable us to determine the disk scale height at large distances from the central star. However, the assumption of axisymmetry and uniform illumination is clearly inadequate for this variable source. In addition to producing flux images, our radiation-transfer simulations predict the spatially resolved polarization structure of HH 30. We have also performed k'-band simulations for HH 30 in anticipation of high-resolution infrared imaging polarimetry.</p