2,070 research outputs found
Mycobacterium tuberculosis complex DNA from an extinct bison dated 17,000 years before the present
Reply to: Misinterpretation of solid sphere equivalent refractive index measurements and smallest detectable diameters of extracellular vesicles by flow cytometry
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
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