1,779 research outputs found
Magnetic-field measurements of T Tauri stars in the Orion Nebula cluster
We present an analysis of high-resolution () infrared K-band
echelle spectra of 14 T Tauri stars in the Orion Nebula Cluster. We model
Zeeman broadening in three magnetically sensitive \ion{Ti}{1} lines near $2.2\
\mu$m and consistently detect kilogauss-level magnetic fields in the stellar
photospheres. The data are consistent in each case with the entire stellar
surface being covered with magnetic fields, suggesting that magnetic pressure
likely dominates over gas pressure in the photospheres of these stars. These
very strong magnetic fields might themselves be responsible for the
underproduction of X-ray emission of T Tauri stars relative to what is expected
based on main-sequence star calibrations. We combine these results with
previous measurements of 14 stars in Taurus and 5 stars in the TW Hydrae
association to study the potential variation of magnetic-field properties
during the first 10 million years of stellar evolution, finding a steady
decline in total magnetic flux with age.Comment: 34 pages, 17 figures, published in ApJ, 2011, 729, 8
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
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
- âŠ