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

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

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

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

Observational Constraints on Interstellar Grain Alignment

We present new multicolor photo-polarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA and Taurus clouds, we show that the wavelength of maximum polarization (lambda_max) is linearly correlated with the radiation environment of the grains. Using Far-Infrared emission data, we show that the large scatter seen in previous studies of lambda_max as a function of A_V is primarily due to line of sight effects causing some A_V measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in lambda_max vs. A_V, for the individual clouds, are consistent with a common value, while the zero intercepts scale with the average values of the ratios of total-to-selective extinction (R_V) for the individual clouds. Within each cloud we do not find direct correlations between lambda_max and R_V. The positive slope in consistent with recent developments in theory and indicating alignment driven by the radiation field. The present data cannot conclusively differentiate between direct radiative torques and alignment driven by H_2 formation. However, the small values of lambda_max(A_V=0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the lambda_max vs. A_V relation is found to be associated with the characteristics of the embedded Young Stellar Objects (YSO) in the clouds. We propose that this is partially due to locally increased plasma damping of the grain rotation caused by X-rays from the YSOs.Comment: Accepted for publication in the Astrophysical Journa

Near-Infrared Spectroscopy of McNeil's Nebula Object

We present 0.8-5.2 micron spectroscopy of the compact source at the base of a variable nebula (McNeil's Nebula Object) in the Lynds 1630 dark cloud that went into outburst in late 2003. The spectrum of this object reveals an extremely red continuum, CO bands at 2.3-2.5 microns in emission, a deep 3.0 micron ice absorption feature, and a solid state CO absorption feature at 4.7 microns. In addition, emission lines of H, Ca II, Mg I, and Na I are present. The Paschen lines exhibit P Cygni profiles, as do two lines of He I, although the emission features are very weak in the latter. The Brackett lines, however, are seen to be purely in emission. The P Cygni profiles clearly indicate that mass outflow is occurring in a wind with a velocity of ~400 km/s. The H line ratios do not yield consistent estimates of the reddening, nor do they agree with the extinction estimated from the ice feature (A_V ~ 11). We propose that these lines are optically thick and are produced in a dense, ionized wind. The near-infrared spectrum does not appear similar to any known FUor or EXor object. However, all evidence suggests that McNeil's Nebula Object is a heavily-embedded low-mass Class I protostar, surrounded by a disk, whose brightening is due to a recent accretion event.Comment: 11 pages, 2 ps figures, accepted for publication in ApJ Letter

The Relationship between the Optical Depth of the 9.7 micron Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds

We have examined the relationship between the optical depth of the 9.7 micron silicate absorption feature (tau_9.7) and the near-infrared color excess, E(J-Ks) in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J-Ks)=0.3 to 10 mag (corresponding to visual extinction between about 2 and 60 mag.). All lines of sight show the 9.7 micron silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 micron silicate feature optical depth and the extinction (Av) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, tau_9.7 is not a good measure of total dust column density. With few exceptions, the measured tau_9.7 values fall well below the diffuse ISM correlation line for E(J-Ks) > 2 mag (Av >12 mag). Grain growth via coagulation is a likely cause of this effect.Comment: 11 pages including 2 figures, 1 table. Accepted for publication in ApJ Letters, 23 July 200