825 research outputs found
Mechanisms of growth cone repulsion
Research conducted in the last century suggested that chemoattractants guide cells or their processes to appropriate locations during development. Today, we know that many of the molecules involved in cellular guidance can act as chemorepellents that prevent migration into inappropriate territories. Here, we review some of the early seminal experiments and our current understanding of the underlying molecular mechanisms
Using Lorentz forces to control the distribution of bubbles in a vertical tube filled with liquid metal
In this work, a method to increase the residence time of bubbles in tubes or pipes filled with liquid metal is investigated. Imposing a horizontal electric current and a perpendicular horizontal magnetic field generates an upward-directed Lorentz force. This force can counteract gravity and cause floating of bubbles. Even with homogeneous electric fields these float in the mean but fluctuate randomly within the swarm due to mutual interactions. In the present case the cylindrical shape of the container furthermore creates inhomogeneous electric currents and an inhomogeneous force distribution resulting in a macroscopic convection pattern stirring the bubbles and further homogenising the spatial distribution of the bubbles.LIMTECHDFG/HE 7529/1-
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM513-46546
[Abridged] We present the first simultaneous radio, X-ray, ultraviolet, and
optical spectroscopic observations of the M8.5 dwarf TVLM513-46546, with a
duration of 9 hours. These observations are part of a program to study the
origin of magnetic activity in ultracool dwarfs, and its impact on
chromospheric and coronal emission. Here we detect steady quiescent radio
emission superposed with multiple short-duration, highly polarized flares;
there is no evidence for periodic bursts previously reported for this object,
indicating their transient nature. We also detect soft X-ray emission, with
L_X/L_bol~10^-4.9, the faintest to date for any object later than M5, and a
possible weak X-ray flare. TVLM513-46546 continues the trend of severe
violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4
orders of magnitude. From the optical spectroscopy we find that the Balmer line
luminosity exceeds the X-ray luminosity by a factor of a few, suggesting that,
unlike in early M dwarfs, chromospheric heating may not be due to coronal X-ray
emission. More importantly, we detect a sinusoidal H-alpha light curve with a
period of 2 hr, matching the rotation period of TVLM513-46546. This is the
first known example of such Balmer line behavior, which points to a co-rotating
chromospheric hot spot or an extended magnetic structure, with a covering
fraction of about 50%. This feature may be transitory based on the apparent
decline in light curve peak during the four observed maxima. From the radio
data we infer a large scale steady magnetic field of ~100 G, in good agreement
with the value required for confinement of the X-ray emitting plasma. The radio
flares, on the other hand, are produced in a component of the field with a
strength of ~3 kG and a likely multi-polar configuration.Comment: 13 pages, 4 figure
Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB10 and LSR1835+32 and the Possible Role of Rotation
[Abridged] As part of our on-going investigation of magnetic activity in
ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical
observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations
of VB10 (M8), both with a duration of about 9 hours. LSR1835+32 exhibits
persistent radio emission and H-alpha variability on timescales of ~0.5-2 hr.
The detected UV flux is consistent with photospheric emission, and no X-ray
emission is detected to a deep limit of L_X/L_bol<10^-5.7. The H-alpha and
radio emission are temporally uncorrelated, and the ratio of radio to X-ray
luminosity exceeds the correlation seen in F-M6 stars by >2x10^4. Similarly,
L_Halpha/L_X>10 is at least 30 times larger than in early M dwarfs, and
eliminates coronal emission as the source of chromospheric heating. The lack of
radio variability during four rotations of LSR1835+32 requires a uniform
stellar-scale field of ~10 G, and indicates that the H-alpha variability is
dominated by much smaller scales, <10% of the chromospheric volume. VB10, on
the other hand, shows correlated flaring and quiescent X-ray and UV emission,
similar to the behavior of early M dwarfs. Delayed and densely-sampled optical
spectra exhibit a similar range of variability amplitudes and timescales to
those seen in the X-rays and UV, with L_Halpha/L_X~1. Along with our previous
observations of the M8.5 dwarf TVLM513-46546 we conclude that late M dwarfs
exhibit a mix of activity patterns, which points to a transition in the
structure and heating of the outer atmosphere by large-scale magnetic fields.
We find that rotation may play a role in generating the fields as evidenced by
a tentative correlation between radio activity and rotation velocity. The X-ray
emission, however, shows evidence for super-saturation at vsini>25 km/s.Comment: Submitted to Ap
The Angular Momentum Evolution of 0.1-10 Msun Stars From the Birthline to the Main Sequence
(Abridged) Projected rotational velocities (vsini) have been measured for a
sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun)
located in the Orion star-forming complex. These measurements have been
supplemented with data from the literature for Orion stars with masses as low
as 0.1 Msun. The primary finding from analysis of these data is that the upper
envelope of the observed values of angular momentum per unit mass (J/M) varies
as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3
Msun. This power law extends smoothly into the domain of more massive stars (3
to 10 Msun), which in Orion are already on the ZAMS. This result stands in
sharp contrast to the properties of main sequence stars, which show a break in
the power law and a sharp decline in J/M with decreasing mass for stars with M
<2 Msun. A second result of our study is that this break is seen already among
the PMS stars in our Orion sample that are on radiative tracks, even though
these stars are only a few million years old. A comparison of rotation rates
seen for stars on either side of the convective-radiative boundary shows that
stars do not rotate as solid bodies during the transition from convective to
radiative tracks.Comment: to appear in Ap
Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae
We present infrared (IR) and optical echelle spectra of the Classical T Tauri
star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis
to fit atomic and molecular absorption lines and determine key stellar
parameters: Teff = 4126 \pm 24 K, log g = 4.84 \pm 0.16, [M/H] = -0.10 \pm
0.12, vsini = 5.8 \pm 0.6 km/s. The IR spectrum is used to look for Zeeman
broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I
lines near 2.2 microns and find the average value of the magnetic field over
the entire surface is 2.61 \pm 0.23 kG. In addition, several nearby
magnetically insensitive CO lines show no excess broadening above that produced
by stellar rotation and instrumental broadening, reinforcing the magnetic
interpretation for the width of the Ti I lines. We carry out extensive tests to
quantify systematic errors in our analysis technique which may result from
inaccurate knowledge of the effective temperature or gravity, finding that
reasonable errors in these quantities produce a 10% uncertainty in the mean
field measurement.Comment: The tar file includes one Tex file and four .eps figures. The paper
is accepted and tentatively scheduled for the ApJ 1 December 2005, v634, 2
issue. ApJ manuscript submission # 6310
Australian climate-carbon cycle feedback reduced by soil black carbon
Annual emissions of carbon dioxide from soil organic carbon are an order of magnitude greater than all anthropogenic carbon dioxide emissions taken together1. Global warming is likely to increase the decomposition of soil organic carbon, and thus the release of carbon dioxide from soils2,3,4,5, creating a positive feedback6,7,8,9. Current models of global climate change that recognize this soil carbon feedback are inaccurate if a larger fraction of soil organic carbon than postulated has a very slow decomposition rate. Here we show that by including realistic stocks of black carbon in prediction models, carbon dioxide emissions are reduced by 18.3 and 24.4% in two Australian savannah regions in response to a warming of 3ââC over 100 years1. This reduction in temperature sensitivity, and thus the magnitude of the positive feedback, results from the long mean residence time of black carbon, which we estimate to be approximately 1,300 and 2,600 years, respectively. The inclusion of black carbon in climate models is likely to require spatially explicit information about its distribution, given that the black carbon content of soils ranged from 0 to 82% of soil organic carbon in a continental-scale analysis of Australia. We conclude that accurate information about the distribution of black carbon in soils is important for projections of future climate change
The FUV spectrum of TW Hya. I. Observations of H Fluorescence
We observed the classical T Tauri star TW Hya with \textit{HST}/STIS using
the E140M grating, from 1150--1700 \AA, with the E230M grating, from 2200--2900
\AA, and with \FUSE from 900--1180 \AA. Emission in 143 Lyman-band H lines
representing 19 progressions dominates the spectral region from 1250--1650 \AA.
The total H emission line flux is erg cm
s, which corresponds to at TW Hya's
distance of 56 pc. A broad stellar \Lya line photoexcites the H from
excited rovibrational levels of the ground electronic state to excited
electronic states. The \ion{C}{2} 1335 \AA doublet, \ion{C}{3} 1175 \AA\
multiplet, and \ion{C}{4} 1550 \AA doublet also electronically excite H.
The velocity shift of the H lines is consistent with the photospheric
radial velocity of TW Hya, and the emission is not spatially extended beyond
the 0\farcs05 resolution of \textit{HST}. The H lines have an intrinsic
FWHM of \kms. One H line is significantly weaker than
predicted by this model because of \ion{C}{2} wind absorption. We also do not
observe any H absorption against the stellar \Lya profile. From these
results, we conclude that the H emission is more consistent with an origin
in a disk rather than in an outflow or circumstellar shell. We also analyze the
hot accretion-region lines (e.g., \ion{C}{4}, \ion{Si}{4}, \ion{O}{6}) of TW
Hya, which are formed at the accretion shock, and discuss some reasons why Si
lines appear significantly weaker than other TR region lines.Comment: accepted by ApJ, 42 pages -- 20 text, 11 figure
Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines
We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists
of spectral ranges 40 A wide centered on 1345, 1400, 1497, 1550, and 1900 A.
These UV spectra show strong SiIV, and CIV emission, and large quantities of
sharp (~40 km/s) H2 lines. All the H2 lines belong to the Lyman band and all
the observed lines are single peaked and optically thin. The averages of all
the H2 lines centroids for each star are negative which may indicate that they
come from an outflow. We interpret the emission in H2 as being due to
fluorescence, mostly by Ly_alpha, and identify seven excitation routes within 4
A of that line. We obtain column densities (10^12 to 10^15 cm^-2) and optical
depths (~1 or less) for each exciting transition. We conclude that the
populations are far from being in thermal equilibrium. We do not observe any
lines excited from the far blue wing of Ly_alpha, which implies that the
molecular features are excited by an absorbed profile. SiIV and CIV (corrected
for H2 emission) have widths of ~200 km/s, and an array of centroids
(blueshifted lines, centered, redshifted). These characteristics are difficult
to understand in the context of current models of the accretion shock. For DR
Tau we observe transient strong blueshifted emission, perhaps the a result of
reconnection events in the magnetosphere. We also see evidence of multiple
emission regions for the hot lines. While CIV is optically thin in most stars
in our sample, SiIV is not. However, CIV is a good predictor of SiIV and H2
emission. We conclude that most of the flux in the hot lines may be due to
accretion processes, but the line profiles can have multiple and variable
components.Comment: 67 pages, 19 figures, Accepted in Ap
A Transiting Planet of a Sun-like Star
A planet transits an 11th magnitude, G1V star in the constellation Corona
Borealis. We designate the planet XO-1b, and the star, XO-1, also known as GSC
02041-01657. XO-1 lacks a trigonometric distance; we estimate it to be 200+-20
pc. Of the ten stars currently known to host extrasolar transiting planets, the
star XO-1 is the most similar to the Sun in its physical characteristics: its
radius is 1.0+-0.08 R_Sun, its mass is 1.0+-0.03 M_Sun, V sini < 3 km/s, and
its metallicity [Fe/H] is 0.015+-0.04. The orbital period of the planet XO-1b
is 3.941534+-0.000027 days, one of the longer ones known. The planetary mass is
0.90+-0.07 M_Jupiter, which is marginally larger than that of other transiting
planets with periods between 3 and 4 days. Both the planetary radius and the
inclination are functions of the spectroscopically determined stellar radius.
If the stellar radius is 1.0+-0.08 R_Sun, then the planetary radius is
1.30+-0.11 R_Jupiter and the inclination of the orbit is 87.7+-1.2 degrees. We
have demonstrated a productive international collaboration between professional
and amateur astronomers that was important to distinguishing this planet from
many other similar candidates.Comment: 31 pages, 9 figures, accepted for part 1 of Ap
- âŠ