106 research outputs found
Entering the Digital Commons: Using Affinity Spaces to Foster Authentic Digital Writing in Online and Traditional Writing Courses
Despite the fact that the field of rhetoric and composition has been closely allied to the digital humanities for many years, instructors in these disciplines often remain on their own in terms of adopting, implementing, and evaluating digital technologies. While theoretical scholarship in digital rhetoric is advancing, instructional practices lag behind. Surveying 72 doctoral-granting rhetoric and composition programs, researchers found innovation in the implementation of new media comes primarily from solitary instructors (Anderson and McKee, 74). This article presents several ways in which writing instructors can leverage digital spaces to improve their pedagogies. In particular, the article focuses on digital spaces that James Gee calls “affinity spaces”. While Gee’s notion of affinity spaces often refers to gaming, the concept may be expanded to include virtual spaces that learners visit voluntarily such as blogs, ezines, social media sites, and digital backchannels. By leveraging such spaces, and implementing them using Michelene Chi and Ruth Wylie’s ICAP (Interactive, Constructive, Active, and Passive) framework, writing instructors can construct powerful learning environments. These digital spaces are not only part and parcel of the digital humanities; they are prime territory for engaging students in rhetorical processes – whether analyzing rhetorical messages or generating rhetorical artifacts
LXD: Ten Critical Differences Between LX and UX
The term “Learner Experience Design” is beginning to gain currency. Yet, there is little agreement over what that term means. Is it just user experience design for learners? In my opinion, LX design differs from UX design in ten important ways. Taken together, these differences make the job of learning experience designers quite distinct from the job of user experience designers
Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
Chemistry plays an important role in the structure and evolution of
protoplanetary disks, with implications for the composition of comets and
planets. This is the first of a series of papers based on data from DISCS, a
Submillimeter Array survey of the chemical composition of protoplanetary disks.
The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau
and MWC 480) range in stellar spectral type from M1 to A4 and offer an
opportunity to test the effects of stellar luminosity on the disk chemistry.
The disks were observed in 10 different lines at ~3" resolution and an rms of
~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2,
CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for
HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO
3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN
3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is
no obvious difference between the T Tauri and Herbig Ae sources with regard to
CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only
toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack
cold regions for long enough timescales to allow for efficient deuterium
chemistry, CO freeze-out, and grain chemistry.Comment: 29 pages, 4 figures, accepted for publication in Ap
Chemistry of a Protoplanetary Disk with Grain Settling and Ly_ Radiation
We present results from a model of the chemical evolution of protoplanetary disks. In our models, we directly calculate the changing propagation and penetration of a high energy radiation field with Ly_ radiation included. We also explore the effect on our models of including dust grain settling. We find that, in agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and a correspondingly larger molecular layer, which leads to an increase in column density for molecular species such as CO, CN, and SO. The inclusion of Ly_ radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216 Å. These include HCN, NH 3 , and CH 4 , for which the column densities are decreased by an order of magnitude or more due to the presence of Ly_ radiation in the UV spectrum. A few species, such as CO 2 and SO, are enhanced by the presence of Ly_ radiation, but rarely by more than a factor of a few.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90767/1/0004-637X_726_1_29.pd
Disk Imaging Survey of Chemistry with SMA: II. Southern Sky Protoplanetary Disk Data and Full Sample Statistics
This is the second in a series of papers based on data from DISCS, a
Submillimeter Array observing program aimed at spatially and spectrally
resolving the chemical composition of 12 protoplanetary disks. We present data
on six Southern sky sources - IM Lup, SAO 206462 (HD 135344b), HD 142527, AS
209, AS 205 and V4046 Sgr - which complement the six sources in the Taurus star
forming region reported previously. CO 2-1 and HCO+ 3-2 emission are detected
and resolved in all disks and show velocity patterns consistent with Keplerian
rotation. Where detected, the emission from DCO+ 3-2, N2H+ 3-2, H2CO 3-2 and
4-3,HCN 3-2 and CN 2-1 are also generally spatially resolved. The detection
rates are highest toward the M and K stars, while the F star SAO 206462 has
only weak CN and HCN emission, and H2CO alone is detected toward HD 142527.
These findings together with the statistics from the previous Taurus disks,
support the hypothesis that high detection rates of many small molecules depend
on the presence of a cold and protected disk midplane, which is less common
around F and A stars compared to M and K stars. Disk-averaged variations in the
proposed radiation tracer CN/HCN are found to be small, despite two orders of
magnitude range of spectral types and accretion rates. In contrast, the
resolved images suggest that the CN/HCN emission ratio varies with disk radius
in at least two of the systems. There are no clear observational differences in
the disk chemistry between the classical/full T Tauri disks and transitional
disks. Furthermore, the observed line emission does not depend on measured
accretion luminosities or the number of infrared lines detected, which suggests
that the chemistry outside of 100 AU is not coupled to the physical processes
that drive the chemistry in the innermost few AU.Comment: accepted for publication in ApJ, 41 pages including 7 figure
Far-Ultraviolet H_2 Emission from Circumstellar Disks
We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauri stars (CTTS), including 20 new spectra obtained with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope. Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or Orion Molecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex, and 1, TW Hya, in the 10 Myr old TW Hydrae Association. We also obtained FUV ACS/SBC spectra of 10 non-accreting sources surrounded by debris disks with ages between 10 and 125 Myr. We use a feature in the FUV spectra due mostly to electron impact excitation of H_2 to study the evolution of the gas in the inner disk. We find that the H_2 feature is absent in non-accreting sources, but is detected in the spectra of CTTS and correlates with accretion luminosity. Since all young stars have active chromospheres which produce strong X-ray and UV emission capable of exciting H_2 in the disk, the fact that the non-accreting sources show no H_2 emission implies that the H_2 gas in the inner disk has dissipated in the non-accreting sources, although dust (and possibly gas) remains at larger radii. Using the flux at 1600 Å, we estimate that the column density of H_2 left in the inner regions of the debris disks in our sample is less than ~3 × 10^(–6) g cm^(-2), 9 orders of magnitude below the surface density of the minimum mass solar nebula at 1 AU
The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems I: Electron-Impact H2 and Accretion Shocks
We present deep spectroscopic observations of the classical T Tauri stars DF
Tau and V4046 Sgr in order to better characterize two important sources of
far-ultraviolet continuum emission in protoplanetary disks. These new Hubble
Space Telescope-Cosmic Origins Spectrograph observations reveal a combination
of line and continuum emission from collisionally excited H2 and emission from
accretion shocks. H2 is the dominant emission in the 1400-1650 A band spectrum
of V4046 Sgr, while an accretion continuum contributes strongly across the
far-ultraviolet spectrum of DF Tau. We compare the spectrum of V4046 Sgr to
models of electron-impact induced H2 emission to constrain the physical
properties of the emitting region, after making corrections for attenuation
within the disk. We find reasonable agreement with the broad spectral
characteristics of the H2 model, implying N(H2) ~ 10^{18} cm^{-2}, T(H2) =
3000^{+1000}_{-500} K, and a characteristic electron energy in the range of ~
50 - 100 eV. We propose that self-absorption and hydrocarbons provide the
dominant attenuation for H2 line photons originating within the disk. For both
DF Tau and V4046 Sgr, we find that a linear fit to the far-UV data can
reproduce near-UV/optical accretion spectra. We discuss outstanding issues
concerning how these processes operate in protostellar/protoplanetary disks,
including the effective temperature and absolute strength of the radiation
field in low-mass protoplanetary environments. We find that the 912-2000A
continuum in low-mass systems has an effective temperature of ~10^{4} K with
fluxes 10^{5-7} times the interstellar level at 1 AU.Comment: 14 pages, 8 figures, 3 tables. ApJ, accepte
The Far-Ultraviolet "Continuum" in Protoplanetary Disk Systems II: CO Fourth Positive Emission and Absorption
We exploit the high sensitivity and moderate spectral resolution of the
-Cosmic Origins Spectrograph to detect far-ultraviolet spectral features
of carbon monoxide (CO) present in the inner regions of protoplanetary disks
for the first time. We present spectra of the classical T Tauri stars HN Tau,
RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN
Tau shows CO bands in absorption against the accretion continuum. We measure a
CO column density and rotational excitation temperature of N(CO) = 2 +/- 1
10 cm and T_rot(CO) 500 +/- 200 K for the absorbing gas.
We also detect CO A-X band emission in RECX-11 and V4046 Sgr, excited by
ultraviolet line photons, predominantly HI LyA. All three objects show emission
from CO bands at 1560 \AA, which may be excited by a combination
of UV photons and collisions with non-thermal electrons. In previous
observations these emission processes were not accounted for due to blending
with emission from the accretion shock, collisionally excited H, and
photo-excited H2; all of which appeared as a "continuum" whose components could
not be separated. The CO emission spectrum is strongly dependent upon the shape
of the incident stellar LyA emission profile. We find CO parameters in the
range: N(CO) 10 cm, T_{rot}(CO) > 300 K for the LyA-pumped
emission. We combine these results with recent work on photo- and
collisionally-excited H emission, concluding that the observations of
ultraviolet-emitting CO and H2 are consistent with a common spatial origin. We
suggest that the CO/H2 ratio in the inner disk is ~1, a transition between the
much lower interstellar value and the higher value observed in solar system
comets today, a result that will require future observational and theoretical
study to confirm.Comment: 12 pages, 7 figures, 3 tables. ApJ - accepte
A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars
We present a far-ultraviolet (FUV) spectral atlas consisting of spectra of 91
pre-main sequence stars. Most stars in this sample were observed with the Space
Telescope Imaging Spectrograph (STIS) and Advanced Camera for Surveys (ACS) on
the \emph{Hubble Space Telescope} (\emph{HST}). We find strong correlations
among the \ion{O}{1} 1304 triplet, %\ion{C}{2} 1335, the
\ion{Si}{4} 1394/1403 doublet, the \ion{C}{4} 1549
doublet, and the \ion{He}{2} 1640 line luminosities. For classical T
Tauri stars (CTTSs), we also find strong correlations between these lines and
the accretion luminosity, suggesting that these lines form in processes related
to accretion. These FUV line fluxes and X-ray luminosity correlate loosely with
large scatters. The FUV emission also correlates well with H, H,
and \ion{Ca}{2} K line luminosities. These correlations between FUV and optical
diagostics can be used to obtain rough estimates of FUV line fluxes from
optical observations. Molecular hydrogen (H) emission is generally
present in the spectra of actively accreting CTTSs but not the weak-lined T
Tauri stars (WTTSs) that are not accreting. The presence of H emission in
the spectrum of HD 98800 N suggests that the disk should be classified as
actively accreting rather than a debris disk. The spectra in the atlas are
available at http://archive.stsci.edu/prepds/ttauriatlas.Comment: 89 pages, 30 figures, published in Ap
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