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

Accretion variability of Herbig Ae/Be stars observed by X-Shooter. HD 31648 and HD 163296

This work presents X-Shooter/VLT spectra of the prototypical, isolated Herbig Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by timescales ranging from days to months. Each spectrum spans over a wide wavelength range covering from 310 to 2475 nm. We have monitored the continuum excess in the Balmer region of the spectra and the luminosity of twelve ultraviolet, optical and near infrared spectral lines that are commonly used as accretion tracers for T Tauri stars. The observed strengths of the Balmer excesses have been reproduced from a magnetospheric accretion shock model, providing a mean mass accretion rate of 1.11 x 10^-7 and 4.50 x 10^-7 Msun yr^-1 for HD 31648 and HD 163296, respectively. Accretion rate variations are observed, being more pronounced for HD 31648 (up to 0.5 dex). However, from the comparison with previous results it is found that the accretion rate of HD 163296 has increased by more than 1 dex, on a timescale of ~ 15 years. Averaged accretion luminosities derived from the Balmer excess are consistent with the ones inferred from the empirical calibrations with the emission line luminosities, indicating that those can be extrapolated to HAe stars. In spite of that, the accretion rate variations do not generally coincide with those estimated from the line luminosities, suggesting that the empirical calibrations are not useful to accurately quantify accretion rate variability.Comment: 14 pages, 7 Figures, Accepted in Ap

Sources for Neo-satirical Drawings: Surrealist and Conceptual Processes

This study was concerned with social conunent and the intensity of emotional expression. The work was not intended to be a sociological study with illustrations, nor as an over-literate approach to art, but rather as a study of fine art problems. The problems and questions raised were as follows: (1) How are contradictions resolved when strict definitions of fine art and caricature are combined in drawing? (2) How are surrealist and conceptual processes integrated with biomorphic and mechanical forms? (2) What are the possible symbols and techniques for expressing ideas and emotions about the mechano-organic theme? This study was intended to help beginning art students develop a sense of draftsmanship, and to expose them to diverse sources for content statements. The thought processes, techniques and personal experiences, which were sources for these drawings could serve as departure points for students to generate original ideas, especially for satirical drawing. As an adjunct to this study one was able to expand the creative and technical aspects of drawing in a neosatirical mode of expression