858 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
The [Ne III] Jet of DG Tau and its Ionization Scenarios
Forbidden neon emission from jets of low-mass young stars can be used to
probe the underlying high-energy processes in these systems. We analyze spectra
of the jet of DG Tau obtained with the Very Large Telescope/X-Shooter
spectrograph in 2010. [Ne III] 3869 is clearly detected in the
innermost 3" microjet and the outer knot located at 6".5. The velocity
structure of the inner microjet can be decomposed into the low-velocity
component (LVC) at km/s and the high-velocity component (HVC) at
km/s. Based on the observed [Ne III] flux and its spatial extent,
we suggest the origins of the [Ne III] emission regions and their relation with
known X-ray sources along the jet. The flares from the hard X-ray source close
to the star may be the main ionization source of the innermost microjet. The
fainter soft X-ray source at 0".2 from the star may provide sufficient heating
to help to sustain the ionization fraction against the recombination in the
flow. The outer knot may be reionized by shocks faster than 100 km/s such that
[Ne III] emission reappears and that the soft X-ray emission at 5".5 is
produced. Velocity decomposition of the archival Hubble Space Telescope spectra
obtained in 1999 shows that the HVC had been faster, with a velocity centroid
of km/s. Such a decrease in velocity may potentially be explained
by the expansion of the stellar magnetosphere, changing the truncation radius
and thus the launching speed of the jet. The energy released by magnetic
reconnections during relaxation of the transition can heat the gas up to
several tens of megakelvin and provide the explanation for on-source keV X-ray
flares that ionize the neon microjet
Velocity-Resolved [Ne III] from X-Ray Irradiated Sz 102 Microjets
Neon emission lines are good indicators of high-excitation regions close to a
young stellar system because of their high ionization potentials and large
critical densities. We have discovered [Ne III]{\lambda}3869 emission from the
microjets of Sz 102, a low-mass young star in Lupus III. Spectroastrometric
analyses of two-dimensional [Ne III] spectra obtained from archival
high-dispersion () Very Large Telescope/UVES data suggest that
the emission consists of two velocity components spatially separated by ~ 0."3,
or a projected distance of ~ 60 AU. The stronger redshifted component is
centered at ~ +21 km/s with a line width of ~ 140 km/s, and the weaker
blueshifted component at ~ -90 km/s with a line width of ~ 190 km/s. The two
components trace velocity centroids of the known microjets and show large line
widths that extend across the systemic velocity, suggesting their potential
origins in wide-angle winds that may eventually collimate into jets. Optical
line ratios indicate that the microjets are hot ( K)
and ionized ( cm). The blueshifted component
has ~ 13% higher temperature and ~ 46% higher electron density than the
redshifted counterpart, forming a system of asymmetric pair of jets. The
detection of the [Ne III]{\lambda}3869 line with the distinct velocity profile
suggests that the emission originates in flows that may have been strongly
ionized by deeply embedded hard X-ray sources, most likely generated by
magnetic processes. The discovery of [Ne III]{\lambda}3869 emission along with
other optical forbidden lines from Sz 102 support the picture of wide-angle
winds surrounding magnetic loops in the close vicinity of the young star.
Future high sensitivity X-ray imaging and high angular-resolution optical
spectroscopy may help confirm the picture proposed.Comment: 33 pages, 9 figures, 2 tables; accepted for publication in the ApJ
(minor typo and reference list fixed
Complete 1-Loop Calculation of the T-violating D-Parameter in Neutron Decay in the MSSM
We investigate the violation of time reversal invariance in the decay of the
free neutron in the framework of the Minimal Supersymmetric Standard Model
(MSSM). The coefficient of the triple product of the neutron spin and the
momenta of electron and neutrino, the so called D parameter, is computed at one
loop order including all diagrams. We find that D is mainly sensitive to the
trilinear A coupling in the squark sector and to the phase of the coefficient
\mu which mixes the two Higgs superfields. The maximal MSSM contribution using
parameters still allowed by experiment is however at D \approx 10^{-7}, while
QED final state interactions give a value of D_{fsi} = - 2.3 * 10^{-5}.
Explicit expressions for all relevant diagrams are given in an Appendix.Comment: 24 pages, 14 figures; Improved scanning of parameter space leads to
some numerical change, but the basic conclusion remains unchange
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 à 10^4 K
HST FUV C IV observations of the hot DG Tauri jet
Protostellar jets are tightly connected to the accretion process and regulate
the angular momentum balance of accreting star-disk systems. The DG Tau jet is
one of the best-studied protostellar jets and contains plasma with temperatures
ranging over three orders of magnitude within the innermost 50 AU of the jet.
We present new Hubble Space Telescope (HST) far ultraviolet (FUV) long-slit
spectra spatially resolving the C IV emission (T~1e5 K) from the jet for the
first time, and quasi-simultaneous HST observations of optical forbidden
emission lines ([O I], [N II], [S II] and [O III]) and fluorescent H2 lines.
The C IV emission peaks at 42 AU from the stellar position and has a FWHM of 52
AU along the jet. Its deprojected velocity of around 200 km/s decreases
monotonically away from the driving source. In addition, we compare our HST
data with the X-ray emission from the DG Tau jet. We investigate the
requirements to explain the data by an initially hot jet compared to local
heating. Both scenarios indicate a mass loss by the T~1e5 K jet of ~1e-9
Msun/year, i.e., between the values for the lower temperature jet (T~1e4 K) and
the hotter X-ray emitting part (T>1e6 K). However, a simple initially hot wind
requires a large launching region (~1 AU), and we therefore favor local
heating.Comment: 5 pages, 2 figures, accepted by A&A letter
Pulsed Accretion in the T Tauri Binary TWA 3A
TWA 3A is the most recent addition to a small group of young binary systems
that both actively accrete from a circumbinary disk and have spectroscopic
orbital solutions. As such, it provides a unique opportunity to test binary
accretion theory in a well-constrained setting. To examine TWA 3A's
time-variable accretion behavior, we have conducted a two-year, optical
photometric monitoring campaign, obtaining dense orbital phase coverage (~20
observations per orbit) for ~15 orbital periods. From U-band measurements we
derive the time-dependent binary mass accretion rate, finding bursts of
accretion near each periastron passage. On average, these enhanced accretion
events evolve over orbital phases 0.85 to 1.05, reaching their peak at
periastron. The specific accretion rate increases above the quiescent value by
a factor of ~4 on average but the peak can be as high as an order of magnitude
in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in
good agreement with numerical simulations of binary accretion with similar
orbital parameters. In these simulations, periastron accretion bursts are
fueled by periodic streams of material from the circumbinary disk that are
driven by the binary orbit. We find that TWA 3A's average accretion behavior is
remarkably similar to DQ Tau, another T Tauri binary with similar orbital
parameters, but with significantly less variability from orbit to orbit. This
is only the second clear case of orbital-phase-dependent accretion in a T Tauri
binary.Comment: 6 pages, 4 figure
Doppler Probe of Accretion onto a T Tauri star
The YY Ori stars are T Tauri stars with prominent time-variable redshifted
absorption components that flank certain emission lines. One of the brightest
in this class is S CrA, a visual double star. We have obtained a series of
high-resolution spectra of the two components during four nights with the UVES
spectrograph at the Very Large Telescope. We followed the spectral changes
occurring in S CrA to derive the physical structure of the accreting gas.
We found that both stars are very similar with regard to surface temperature,
radius, and mass. Variable redshifted absorption components are particularly
prominent in the SE component. During one night, this star developed a spectrum
unique among the T Tauri stars: extremely strong and broad redshifted
absorption components appeared in many lines of neutral and ionized metals, in
addition to those of hydrogen and helium. The absorption depths of cooler, low
ionization lines peak at low velocities - while more highly ionized lines have
peak absorption depths at high velocities. The different line profiles indicate
that the temperature and density of the accretion stream increase as material
approaches the star. We derive the physical conditions of the flow at several
points along the accretion funnel directly from the spectrum of the infalling
gas. We estimated mass accretion rates of about 10^(-7) solar masses per year,
which is similar to that derived from the relation based on the strength of H
alpha emission line.
This is the first time the density and temperature distributions in accretion
flows around a T Tauri star have been inferred from observations. Compared with
predictions from standard models of accretion in T Tauri stars, which assume a
dipole stellar magnetic field, we obtained higher densities and a steeper
temperature rise toward the star.Comment: Replaces 1408.1846 4 pages, 4 figures. Appears in Astronomy and
Astrophysics, 201
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