7,183 research outputs found
Spatially and Spectrally Resolved Hydrogen Gas within 0.1 AU of T Tauri and Herbig Ae/Be Stars
We present near-infrared observations of T Tauri and Herbig Ae/Be stars with
a spatial resolution of a few milli-arcseconds and a spectral resolution of
~2000. Our observations spatially resolve gas and dust in the inner regions of
protoplanetary disks, and spectrally resolve broad-linewidth emission from the
Brackett gamma transition of hydrogen gas. We use the technique of
spectro-astrometry to determine centroids of different velocity components of
this gaseous emission at a precision orders of magnitude better than the
angular resolution. In all sources, we find the gaseous emission to be more
compact than or distributed on similar spatial scales to the dust emission. We
attempt to fit the data with models including both dust and Brackett
gamma-emitting gas, and we consider both disk and infall/outflow morphologies
for the gaseous matter. In most cases where we can distinguish between these
two models, the data show a preference for infall/outflow models. In all cases,
our data appear consistent with the presence of some gas at stellocentric radii
of ~0.01 AU. Our findings support the hypothesis that Brackett gamma emission
generally traces magnetospherically driven accretion and/or outflows in young
star/disk systems.Comment: 48 pages, including 17 figures. Accepted for publication by Ap
Spatially Resolved Spectroscopy of Sub-AU-Sized Regions of T Tauri and Herbig Ae/Be Disks
We present spatially resolved near-IR spectroscopic observations of 15 young
stars. Using a grism spectrometer behind the Keck Interferometer, we obtained
an angular resolution of a few milli-arcseconds and a spectral resolution of
230, enabling probes of both gas and dust in the inner disks surrounding the
target stars. We find that the angular size of the near-IR emission typically
increases with wavelength, indicating hot, presumably gaseous material within
the dust sublimation radius. Our data also clearly indicate Brackett-gamma
emission arising from hot hydrogen gas, and suggest the presence of water vapor
and carbon monoxide gas in the inner disks of several objects. This gaseous
emission is more compact than the dust continuum emission in all cases. We
construct simple physical models of the inner disk and fit them to our data to
constrain the spatial distribution and temperature of dust and gas emission
components.Comment: 40 pages, 8 figures. Accepted for publication in Ap
Coping with speaker-related variation via abstract phonemic categories
Listeners can cope with considerable variation in the way that different speakers talk. We argue here that they can do so because of a process of phonological abstraction in the speech-recognition system. We review evidence that listeners adjust the bounds of phonemic categories after only very limited exposure to a deviant realisation of a given phoneme. This learning can be talker-specific and is stable over time; further, the learning generalizes to previously unheard words containing the deviant phoneme. Together these results suggest that the learning involves adjustment of prelexical phonemic representations which mediate between the speech signal and the mental lexicon during word recognition. We argue that such an abstraction process is inconsistent with claims made by some recent models of language processing that the mental lexicon consists solely of multiple detailed traces of acoustic episodes. Simulations with a purely episodic model without functional prelexical abstraction confirm that such a model cannot account for the evidence on lexical generalization of perceptual learning. We conclude that abstract phonemic categories form a necessary part of lexical access, and that the ability to store talker-specific knowledge about those categories provides listeners with the means to deal with cross-talker variation
Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer
We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars
with a spatial resolution of a few milliarcseconds and a spectral resolution of
~2000. Our observations span the K-band, and include the Br gamma transition of
Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several
targets we also present data from Keck/NIRSPEC that provide higher spectral
resolution, but a seeing-limited spatial resolution, of the same spectral
features. We analyze the Br gamma emission in the context of both disk and
infall/outflow models, and conclude that the Br gamma emission traces gas at
very small stellocentric radii, consistent with the magnetospheric scale.
However some Br gamma-emitting gas also seems to be located at radii of >0.1
AU, perhaps tracing the inner regions of magnetically launched outflows. CO
emission is detected from several objects, and we generate disk models that
reproduce both the KI and NIRSPEC data well. We infer the CO spatial
distribution to be coincident with the distribution of continuum emission in
most cases. Furthermore the Br gamma emission in these objects is roughly
coincident with both the CO and continuum emission. We present potential
explanations for the spatial coincidence of continuum, Br gamma, and CO
overtone emission, and explore the implications for the low occurrence rate of
CO overtone emission in young stars. Finally, we provide additional discussion
of V1685 Cyg, which is unusual among our sample in showing large differences in
emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA
Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2
We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in
ApJ (April 1, 2001 issue
Opposing and following responses in sensorimotor speech control : why responses go both ways
When talking, speakers continuously monitor and use the auditory feedback of their own voice to control and inform speech production processes. When speakers are provided with auditory feedback that is perturbed in real time, most of them compensate for this by opposing the feedback perturbation. But some speakers follow the perturbation. In the current study, we investigated whether the state of the speech production system at perturbation onset may determine what type of response (opposing or following) is given. The results suggest that whether a perturbation-related response is opposing or following depends on ongoing fluctuations of the production system: It initially responds by doing the opposite of what it was doing. This effect and the non-trivial proportion of following responses suggest that current production models are inadequate: They need to account for why responses to unexpected sensory feedback depend on the production-system’s state at the time of perturbation
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