245 research outputs found
A Consistent Model of the Accretion Shock Region in Classical T Tauri Stars
We develop a consistent model of the accretion shock region in Classical T Tauri Stars (CTTSs). The initial conditions of the post-shock flow are determined by the irradiated shock precursor and the ionization state is calculated without assuming ionization equilibrium. Comparison with observations of the C IV resonance lines (λλ 1550 Ă
) for CTTSs indicate that the post-shock emission predicted by the model is too large, for a reasonable range of parameters. If the model is to reproduce the observations, C IV emission from CTTSs has to be dominated by pre-shock emission, for stars with moderate to large accretion rates. For stars with low accretion rates, the observations suggest a comparable contribution between the pre- and post-shock regions. These conclusions are consistent with previous results indicating that the post-shock will be buried under the stellar photosphere for moderate to large accretion rates
Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data
We present theoretical atmosphere, spectral, and light-curve models for
extrasolar giant planets (EGPs) undergoing strong irradiation for which {\it
Spitzer} planet/star contrast ratios or light curves have been published (circa
June 2007). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD
179949b, and And b. By comparing models with data, we find that a
number of EGP atmospheres experience thermal inversions and have stratospheres.
This is particularly true for HD 209458b, HD 149026b, and And b.
This finding translates into qualitative changes in the planet/star contrast
ratios at secondary eclipse and in close-in EGP orbital light curves. Moreover,
the presence of atmospheric water in abundance is fully consistent with all the
{\it Spitzer} data for the measured planets. For planets with stratospheres,
water absorption features invert into emission features and mid-infrared fluxes
can be enhanced by a factor of two. In addition, the character of near-infrared
planetary spectra can be radically altered. We derive a correlation between the
importance of such stratospheres and the stellar flux on the planet, suggesting
that close-in EGPs bifurcate into two groups: those with and without
stratospheres. From the finding that TrES-1 shows no signs of a stratosphere,
while HD 209458b does, we estimate the magnitude of this stellar flux
breakpoint. We find that the heat redistribution parameter, P, for the
family of close-in EGPs assumes values from 0.1 to 0.4. This paper
provides a broad theoretical context for the future direct characterization of
EGPs in tight orbits around their illuminating stars.Comment: Accepted to Ap. J., provided here in emulateapj format: 28 pages, 8
figures, many with multiple panel
Non-detection of the OH Meinel system in comet P/Swift-Tuttle
We report a search for emissions from the OH Meinel system in high-resolution near-infrared spectra of comet P/Swift-Tuttle. Because of the large cometary heliocentric velocity and high resolution of the spectrograph, the cometary lines should be well separated from the bright OH sky lines. Contrary to the findings of Tozzi et al. (1994) - who report seeing cometary OH at intensities comparable to the sky emissions in their low-resolution spectra - we find no OH in these spectra with an upper limit of 5% the value of the night sky lines. The non-detection of these cometary lines is consistent with theoretical calculations of expected emission strengths from prompt and fluorescent emission from cometary OH
A high-Resolution Catalog of Cometary Emission Lines
Using high-resolution spectra obtained with the Hamilton echelle spectrograph at Lick Observatory, we have constructed a catalog of emission lines observed in comets Swift-Tuttle and Brorsen-Metcalf. The spectra cover the range between 3800 Ă
and 9900 Ă
with a spectral resolution of λ/Îλ~42000. In the spectra, we catalog 2997 emission lines of which we identify 2438. We find cometary lines due to H, O, C_2, CN, NH_2, C_3, H_2O^+, CH, and CH^+. We list 559 unidentified lines compiled from the two spectra and comment on possibilities for their origins
A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau
In searches for low-mass companions to late-type stars, correlation between
radial velocity variations and line bisector slope changes indicates
contamination by large starspots. Two young stars demonstrate that this test is
not sufficient to rule out starspots as a cause of radial velocity variations.
As part of our survey for substellar companions to T Tauri stars, we identified
the ~2 Myr old planet host candidates DN Tau and V836 Tau. In both cases,
visible light radial velocity modulation appears periodic and is uncorrelated
with line bisector span variations, suggesting close companions of several
M_Jup in these systems. However, high-resolution, infrared spectroscopy shows
that starspots cause the radial velocity variations. We also report unambiguous
results for V827 Tau, identified as a spotted star on the basis of both visible
light and infrared spectroscopy. Our results suggest that infrared follow up
observations are critical for determining the source of radial velocity
modulation in young, spotted stars.Comment: Accepted for publication in the Astrophysical Journal Letter
The Far-Ultraviolet Spectra of TW Hya. II. Models of H2 Fluorescence in a Disk
We measure the temperature of warm gas at planet-forming radii in the disk
around the classical T Tauri star (CTTS) TW Hya by modelling the H2
fluorescence observed in HST/STIS and FUSE spectra. Strong Ly-alpha emission
irradiates a warm disk surface within 2 AU of the central star and pumps
certain excited levels of H2. We simulate a 1D plane-parallel atmosphere to
estimate fluxes for the 140 observed H2 emission lines and to reconstruct the
Ly-alpha emission profile incident upon the warm H2. The excitation of H2 can
be determined from relative line strengths by measuring self-absorption in
lines with low-energy lower levels, or by reconstructing the Ly-alpha profile
incident upon the warm H2 using the total flux from a single upper level and
the opacity in the pumping transition. Based on those diagnostics, we estimate
that the warm disk surface has a column density of log
N(H2)=18.5^{+1.2}_{-0.8}, a temperature T=2500^{+700}_{-500} K, and a filling
factor of H2, as seen by the source of Ly-alpha emission, of 0.25\pm0.08 (all
2-sigma error bars). TW Hya produces approximately 10^{-3} L_\odot in the FUV,
about 85% of which is in the Ly-alpha emission line. From the H I absorption
observed in the Ly-alpha emission, we infer that dust extinction in our line of
sight to TW Hya is negligible.Comment: Accepted by ApJ. 26 pages, 17 figures, 6 table
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
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
Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem
We compare the angular momentum extracted by a wind from a pre-main-sequence
star to the torques arising from the interaction between the star and its
Keplerian accretion disk. We find that the wind alone can counteract the
spin-up torque from mass accretion, solving the mystery of why accreting
pre-main-sequence stars are observed to spin at less than 10% of break-up
speed, provided that the mass outflow rate in the stellar winds is ~10% of the
accretion rate. We suggest that such massive winds will be driven by some
fraction of the accretion power. For observationally constrained
typical parameters of classical T-Tauri stars, needs to be between a
few and a few tens of percent. In this scenario, efficient braking of the star
will terminate simultaneously with accretion, as is usually assumed to explain
the rotation velocities of stars in young clusters.Comment: Accepted by ApJ Letter
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
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