14,120 research outputs found
How Hot is the Wind from TW Hydrae?
It has recently been suggested that the winds from Classical T Tauri stars in
general, and the wind from TW Hya in particular, reaches temperatures of at
least 300,000 K while maintaing a mass loss rate of \Msol
yr or larger. If confirmed, this would place strong new requirements on
wind launching and heating models. We therefore re-examine spectra from the
Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope and
spectra from the Far Ultraviolet Spectroscopic Explorer satellite in an effort
to better constrain the maximum temperature in the wind of TW Hya. We find
clear evidence for a wind in the \ion{C}{2} doublet at 1037 \AA and in the
\ion{C}{2} multiplet at 1335 \AA. We find no wind absorption in the \ion{C}{4}
1550 \AA doublet observed at the same time as the \ion{C}{2} 1335 \AA line or
in observations of \ion{O}{6} observed simultaneously with the \ion{C}{2} 1037
\AA line. The presence or absence of \ion{C}{3} wind absorption is ambiguous.
The clear lack of a wind in the \ion{C}{4} line argues that the wind from TW
Hya does not reach the 100,000 K characteristic formation temperature of this
line. We therefore argue that the available evidence suggests that the wind
from TW Hya, and probably all classical T Tauri stars, reaches a maximum
temperature in the range of 10,000 -- 30,000 K.Comment: 17 pages, 3 figures, Figure 1 in 2nd version fixes a small velocity
scaling error and new revision adds a reference to an additional paper
recently foun
Gear drive automatically indexes rotary table
Combination indexer and drive unit drills equally spaced circular hole patterns on rotary tables. It automatically rotates the table a distance exactly equal to one hole spacing for each revolution of a special idler gear
The Origins of Fluorescent H_2 Emission From T Tauri Stars
We survey fluorescent H_2 emission in HST STIS spectra of the classical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DG Tau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of those sources we detect between 41 and 209 narrow H_2 emission lines, most of which are pumped by strong Lyα emission. H_2 emission is not detected from the WTTS V410 Tau. The fluorescent H_2 emission appears to be common to circumstellar environments around all CTTSs, but high spectral and spatial resolution STIS observations reveal diverse phenomenon. Blueshifted H_2 emission detected from RU Lupi, T Tau, and DG Tau is consistent with an origin in an outflow. The H_2 emission from TW Hya, DF Tau, and V836 Tau is centered at the radial velocity of the star and is consistent with an origin in a warm disk surface. The H_2 lines from RU Lupi, DF Tau, and T Tau also have excess blueshifted H_2 emission that extends to as much as -100 km s^(-1). The strength of this blueshifted component from DF Tau and T Tau depends on the upper level of the transition. In all cases, the small aperture and attenuation of H_2 emission by stellar winds restricts the H_2 emission to be formed close to the star. In the observation of RU Lupi, the Lyα emission and the H_2 emission that is blueshifted by 15 km s^(-1) are extended to the SW by ~0".07, although the faster H_2 gas that extends to ~100 km s^(-1) is not spatially extended. We also find a small reservoir of H_2 emission from TW Hya and DF Tau consistent with an excitation temperature of ~2.5 × 10^4 K
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
KH 15D: A Spectroscopic Binary
We present the results of a high-resolution spectroscopic monitoring program
of the eclipsing pre-main-sequence star KH 15D that reveal it to be a
single-line spectroscopic binary. We find that the best-fit Keplerian model has
a period P = 48.38 days, which is nearly identical to the photometric period.
Thus, we find the best explanation for the periodic dimming of KH 15D is that
the binary motion carries the currently visible star alternately above and
below the edge of an obscuring cloud. The data are consistent with the models
involving an inclined circumstellar disk, as recently proposed by Winn et al.
(2004) and Chiang & Murray-Clay (2004). We show that the mass ratio expected
from models of PMS evolution, together with the mass constraints for the
visible star, restrict the orbital eccentricity to 0.68 < e < 0.80 and the mass
function to 0.125 < Fm < 0.5 Msun.Comment: 8 pages, 4 figures, 3 tables, accepted for publication in September
AJ. Discussion of rotational velocity deferred to Hamilton, et al. (2004, in
prep). Previously reported vsini value in error; Replaced Table 3 with new
Figure 3; Added new Table 2 showing individual radial velocities w.r.t. each
reference star; Fixed typo in Figure
Deletion of \u3cem\u3eULS1\u3c/em\u3e Confers Damage Tolerance in \u3c/em\u3esgs1\u3c/em\u3e Mutants Through a Top3-dependent D-loop Mediated Fork Restart Pathway
Homologous recombination (HR)-based repair during DNA replication can apparently utilize several partially overlapping repair pathways in response to any given lesion. A key player in HR repair is the Sgs1-Top3-Rmi1 (STR) complex, which is critical for resolving X-shaped recombination intermediates formed following bypass of methyl methanesulfonate (MMS)-induced damage. STR mutants are also sensitive to the ribonucleotide reductase inhibitor, hydroxyurea (HU), but unlike MMS treatment, HU treatment is not accompanied by X-structure accumulation, and it is thus unclear how STR functions in this context. Here we provide evidence that HU-induced fork stalling enlists Top3 prior to recombination intermediate formation. The resistance of sgs1Δ mutants to HU is enhanced by the absence of the putative SUMO (Small Ubiquitin MOdifier)-targeted ubiquitin ligase, Uls1, and we demonstrate that Top3 is required for this enhanced resistance and for coordinated breaks and subsequent d-loop formation at forks stalled at the ribosomal DNA (rDNA) replication fork block (RFB). We also find that HU resistance depends on the catalytic activity of the E3 SUMO ligase, Mms21, and includes a rapid Rad51-dependent restart mechanism that is different from the slow Rad51-independent HR fork restart mechanism operative in sgs1Δ ULS1+ mutants. These data support a model in which repair of HU-induced damage in sgs1Δ mutants involves an error-prone break-induced replication pathway but, in the absence of Uls1, shifts to one that is higher-fidelity and involves the formation of Rad51-dependent d-loops
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
From observations collected with the ESPaDOnS spectropolarimeter, we report
the discovery of magnetic fields at the surface of the mildly accreting
classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in
photospheric lines and in the emission lines formed at the base of the
accretion funnels linking the disc to the protostar, and monitored over the
whole rotation cycle of V2129 Oph. We observe that rotational modulation
dominates the temporal variations of both unpolarized and circularly polarized
line profiles. We reconstruct the large-scale magnetic topology at the surface
of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to
be rather complex, with a dominant octupolar component and a weak dipole of
strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to
the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar
radial field spots located at high latitudes and coinciding with cool dark
polar spots at photospheric level. This large-scale field geometry is unusually
complex compared to those of non-accreting cool active subgiants with moderate
rotation rates. As an illustration, we provide a first attempt at modelling the
magnetospheric topology and accretion funnels of V2129 Oph using field
extrapolation. We find that the magnetosphere of V2129 Oph must extend to about
7R* to ensure that the footpoints of accretion funnels coincide with the
high-latitude accretion spots on the stellar surface. It suggests that the
stellar magnetic field succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of V2129
Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes
typical of those observed in cTTSs.Comment: MNRAS, in press (18 pages, 17 figures
Observations On Chytridiaceous Parasites Of Phanerogams. Xi. A Physoderma On Agropyron Repens
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141220/1/ajb211721.pd
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
According to our current understanding, a subclass of the upper main sequence
chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic.
Nevertheless, chemical inhomogeneities were recently discovered on their
surfaces. At the same time, no global magnetic fields stronger than 1-100 G are
detected by modern studies. The goals of our study are to search for magnetic
field in the HgMn binary system 66 Eri and to investigate chemical spots on the
stellar surfaces of both components. Our analysis is based on high quality
spectropolarimetric time-series observations obtained during 10 consecutive
nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study line profile variability. Chemical spot geometry was
reconstructed using multi-line Doppler imaging. We report a non-detection of
magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field.
Circular polarization profiles also do not indicate any signatures of complex
surface magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the HgMn
component we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these elements
reconstructed with the Doppler imaging technique, show relative underabundance
on the hemisphere facing the secondary component. The contrast of chemical
inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.Comment: 13 pages, 14 figure
Stationarity, soft ergodicity, and entropy in relativistic systems
Recent molecular dynamics simulations show that a dilute relativistic gas
equilibrates to a Juettner velocity distribution if ensemble velocities are
measured simultaneously in the observer frame. The analysis of relativistic
Brownian motion processes, on the other hand, implies that stationary
one-particle distributions can differ depending on the underlying
time-parameterizations. Using molecular dynamics simulations, we demonstrate
how this relativistic phenomenon can be understood within a deterministic model
system. We show that, depending on the time-parameterization, one can
distinguish different types of soft ergodicity on the level of the one-particle
distributions. Our analysis further reveals a close connection between time
parameters and entropy in special relativity. A combination of different
time-parameterizations can potentially be useful in simulations that combine
molecular dynamics algorithms with randomized particle creation, annihilation,
or decay processes.Comment: 4 page
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