1,249 research outputs found
Interstellar Polarization in the Taurus Dark Clouds, Wavelength Dependent Position Angles and Cloud Structure Near TMC-1
We use polarimetric observations of two stars (HD29647, HD283809) in the
general direction of TMC-1 in the Taurus Dark Cloud to investigate grain
properties and cloud structure in this region. We show the data to be
consistent with a simple two-component model, in which general interstellar
polarization in the Taurus Cloud is produced by a widely distributed cloud
component with relatively uniform magnetic field orientation; the light from
stars close to TMC-1 suffers additional polarization arising in one (or more)
subcloud(s) with larger average grain size and different magnetic field
directions compared with the general trend. Towards HD29647, in particular, we
show that the unusually low degree of visual polarization relative to
extinction is due to the presence of distinct cloud components in the line of
sight with markedly different magnetic field orientations. Stokes parameter
calculations allow us to separate out the polarization characteristics of the
individual components. Results are fit with the Serkowski empirical formula to
determine the degree and wavelength of maximum polarization. Whereas lambda_max
values in the widely distributed material are similar to the average (0.55um)
for the diffuse interstellar medium, the subcloud in line of sight to
HD~283809, the most heavily reddened star in our study, has lambda_max approx.
0.73um, indicating the presence of grains about 30% larger than this average.
Our model also predicts detectable levels of circular polarization toward both
HD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical
Journal, vol 48
Detailed Interstellar Polarimetric Properties of the Pipe Nebula at Core Scales
We use R-band CCD linear polarimetry collected for about 12000 background
field stars in 46 fields of view toward the Pipe nebula to investigate the
properties of the polarization across this dark cloud. Based on archival 2MASS
data we estimate that the surveyed areas present total visual extinctions in
the range 0.6 < Av < 4.6. While the observed polarizations show a well ordered
large scale pattern, with polarization vectors almost perpendicularly aligned
to the cloud's long axis, at core scales one see details that are
characteristics of each core. Although many observed stars present degree of
polarization which are unusual for the common interstellar medium, our analysis
suggests that the dust grains constituting the diffuse parts of the Pipe nebula
seem to have the same properties as the normal Galactic interstellar medium.
Estimates of the second-order structure function of the polarization angles
suggest that most of the Pipe nebula is magnetically dominated and that
turbulence is sub-Alvenic. The Pipe nebula is certainly an interesting region
where to investigate the processes prevailing during the initial phases of low
mass stellar formation.Comment: 20 pages, 23 figures, Accepted for The Astrophysical Journa
The 3 micron spectrum of NGC 4565
Researchers spectrum of NGC 4565 is essentially featureless. The absence of the 3.0 micron feature (Tau 3.0 less than 0.05) implies that the extinction to the nucleus does not arise to a significant degree in molecular clouds. Researchers deduce Tau 3.0/A sub V less than 0.01, compared with approx. 0.022 for GC-IRS7. These results support the conclusion (McFadzean et al. 1989) that the 3.0 micron absorption in the GC-IR sources is due to the presence of ice in a (probably single) foreground molecular cloud. The 3.4 micron feature is also weak or absent in the researchers spectrum of NGC 4565 (Tau 3.4 less than or equal to 0.07), hence, Tau 3.4/A sub V less than or equal to 0.016, compared with approx. 0.008 towards GC-IRS7. The absence of the feature in NGC 4565 at the signal-to-noise level of the current observations is consistent with a probable moderate degree of extinction towards the nucleus. The observations of NGC 4565 provide a useful comparison for studies of dust in the Galaxy. Limits have been set on the strengths of the 3.0 and 3.4 micron features in NGC 4565. The absence of 3.0 micron absorption is significant, and supports the view that the feature at this wavelength in the Galactic Centre is due to water-ice absorption in a foreground molecular cloud. The non-detection of the 3.4 micron absorption is less surprising and provides indirect support for the association between this feature and the diffuse interstellar medium. The current spectrum probably represents the best that can be achieved with a single-detector instrument within reasonable integration times. It will clearly be of interest in the future to obtain spectra of higher signal-to-noise, as a positive detection of the 3.4 micron feature in an external galaxy, even at a low level, would be of considerable astrophysical significance
CO Line Emission and Absorption from the HL Tau Disk: Where is all the dust?
We present high-resolution infrared spectra of HL Tau, a heavily embedded
young star. The spectra exhibit broad emission lines of hot CO gas as well as
narrow absorption lines of cold CO gas. The column density for this cooler
material (7.5+/-0.2 x 10^18 cm-2) indicates a large column of absorbing gas
along the line of sight. In dense interstellar clouds, this column density of
CO gas is associated with Av~52 magnitudes. However, the extinction toward this
source (Av~23) suggests that there is less dust along the line of sight than
inferred from the CO absorption data. We discuss three possibilities for the
apparent paucity of dust along the line of sight through the flared disk: 1)
the dust extinction has been underestimated due to differences in circumstellar
grain properties, such as grain agglomeration; 2) the effect of scattering has
been underestimated and the actual extinction is much higher; or (3) the line
of sight through the disk is probing a gas-rich, dust-depleted region, possibly
due to the stratification of gas and dust in a pre-planetary disk.Comment: To be published in The Astrophysical Journa
A Catalog of Background Stars Reddened by Dust in the Taurus Dark Clouds
Normal field stars located behind dense clouds are a valuable resource in
interstellar astrophysics, as they provide continua in which to study phenomena
such as gas-phase and solid-state absorption features, interstellar extinction
and polarization. This paper reports the results of a search for highly
reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All
Sky Survey (2MASS) Point Source Catalog to survey a 50 sq deg area of the cloud
to a limiting magnitude of K = 10.0. Photometry in the 1.2-2.2 micron passbands
from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12
micron) from the Spitzer Space Telescope and the Infrared Astronomical
Satellite to provide effective discrimination between reddened field stars and
young stellar objects (YSOs) embedded in the cloud. Our final catalog contains
248 confirmed or probable background field stars, together with estimates of
their total visual extinctions, which span the range 2-29 mag. We also identify
the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously
unrecognized candidate YSO, based on the presence of infrared emission greatly
in excess of that predicted for a normal reddened photosphere at wavelengths >5
microns
A new Classical T Tauri object at the sub-stellar boundary in Chamaeleon II
We have obtained low- and medium-resolution optical spectra of 20 candidate
young low-mass stars and brown dwarfs in the nearby Chamaeleon II dark cloud,
using the Magellan Baade telescope. We analyze these data in conjunction with
near-infrared photometry from the 2-Micron All Sky Survey. We find that one
target, [VCE2001] C41, exhibits broad H(alpha) emission as well as a variety of
forbidden emission lines. These signatures are usually associated with
accretion and outflow in young stars and brown dwarfs. Our spectra of C41 also
reveal LiI in absorption and allow us to derive a spectral type of M5.5 for it.
Therefore, we propose that C41 is a classical T Tauri object near the
sub-stellar boundary. Thirteen other targets in our sample have continuum
spectra without intrinsic absorption or emission features, and are difficult to
characterize. They may be background giants or foreground field stars not
associated with the cloud or embedded protostars, and need further
investigation. The six remaining candidates, with moderate reddening, are
likely to be older field dwarfs, given their spectral types, lack of lithium
and H(alpha).Comment: Astrophysical Journal, accepted June 19, 200
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