363 research outputs found
IUE spectra of a flare in HR 5110: A flaring RS CVn or Algol system?
Ultraviolet spectra of the RS CVn type binary system HR 5110 were obtained with IUE on May 31, 1979 during a period of intense radio flaring of this star. High temperature transition region lines are present, but are not enhanced above observed quiescent strengths. The similarities of HR 5110 to the Algol system, As Eri, suggest that the 1979 May to June flare may involve mass exchange rather than annihilation of coronal magnetic fields
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
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
Data Processing of Lunar Infrared Measurements at High Spatial and Radiometric Resolution to Obtain Brightness Temperatures Scientific Report No. 9
Data processing of lunar infrared measurements at high spatial and radiometric resolution to obtain brightness temperature
Computing and data processing
The applications of computers and data processing to astronomy are discussed. Among the topics covered are the emerging national information infrastructure, workstations and supercomputers, supertelescopes, digital astronomy, astrophysics in a numerical laboratory, community software, archiving of ground-based observations, dynamical simulations of complex systems, plasma astrophysics, and the remote control of fourth dimension supercomputers
The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199
We determine the D/H ratio in the interstellar medium toward the DO white
dwarf PG0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer
(FUSE), with ground-based support from Keck HIRES. We employ curve of growth,
apparent optical depth and profile fitting techniques to measure column
densities and limits of many other species (H2, NaI, CI, CII, CIII, NI, NII,
OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios
such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring
gas-phase D/H, which is key to understanding Galactic chemical evolution and
comparing it to predictions from Big Bang nucleosynthesis. We find column
densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) =
19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma
errors), with an excitation temperature of 143+-5K. The high HI column density
implies that PG0038+199 lies outside of the Local Bubble; we estimate its
distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is
1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on
the scale of Delta v > 8 km/s based on NaI, but there is marginal evidence for
structure on smaller scales. The D/H value is high compared to the majority of
recent D/H measurements, but consistent with the values for two other
measurements at similar distances. D/O is in agreement with other distant
measurements. The scatter in D/H values beyond ~100pc remains a challenge for
Galactic chemical evolution.Comment: 59 pages, 7 tables, 18 figures (1 standalone), accepted by ApJ v2
minor typos correcte
The deuterium-to-oxygen ratio in the interstellar medium
Because the ionization balances for HI, OI, and DI are locked together by
charge exchange, D/O is an important tracer for the value of the D/H ratio and
for potential spatial variations in the ratio. As the DI and OI column
densities are of similar orders of magnitude for a given sight line,
comparisons of the two values will generally be less subject to systematic
errors than comparisons of DI and HI, which differ by about five orders of
magnitude. Moreover, D/O is additionally sensitive to astration, because as
stars destroy deuterium, they should produce oxygen. We report here the results
of a survey of D/O in the interstellar medium performed with FUSE. We also
compare these results with those for D/N. Together with a few results from
previous missions, the sample totals 24 lines of sight. The distances range
from a few pc to ~2000 pc and log N(DI) from ~13 to ~16 (cm-2). The D/O ratio
is constant in the local interstellar medium out to distances of ~150 pc and
N(DI) ~ 1x10^15 cm-2, i.e. within the Local Bubble. In this region of the
interstellar space, we find D/O = (3.84+/-0.16)x10^-2 (1 sigma in the mean).
The homogeneity of the local D/O measurements shows that the spatial variations
in the local D/H and O/H must be extremely small, if any. A comparison of the
Local Bubble mean value with the few D/O measurements available for low
metallicity quasar sight lines shows that the D/O ratio decreases with cosmic
evolution, as expected. Beyond the Local Bubble we detected significant spatial
variations in the value of D/O. This likely implies a variation in D/H, as O/H
is known to not vary significantly over the distances covered in this study.
Our dataset suggests a present-epoch deuterium abundance below 1x10^-5, i.e.
lower than the value usually assumed, around 1.5x10^-5.Comment: 17 pages, 9 figures, 4 tables, accepted for publication in the
Astrophysical Journa
H-D Substitution in Interstellar Solid Methanol: A Key Route for D Enrichment
Deuterium enrichment of interstellar methanol is reproduced experimentally
for the first time via grain-surface H-D substitution in solid methanol at an
atomic D/H ratio of 0.1. Although previous gas-grain models successfully
reproduce the deuterium enrichments observed in interstellar methanol molecules
(D/H of up to 0.4, compared to the cosmic ratio of , the models
exclusively focus on deuterium fractionation resulting from the successive
addition of atomic hydrogen/deuterium on CO. The mechanism proposed here
represents a key route for deuterium enrichment that reproduces the high
observed abundances of deuterated methanol, including multiple deuterations.Comment: 4 pages, 4 figures, Accepted to the ApJ
The Origins of Fluorescent H2 Emission From T Tauri~Stars
We survey fluorescent H2 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 41-209 narrow H2 emission lines, most of which are pumped by strong
Ly-alpha emission. H2 emission is not detected from the WTTS V410 Tau. The
fluorescent H2 emission appears to be common to circumstellar environments
around all CTTSs, but high spectral and spatial resolution STIS observations
reveal diverse phenomenon. Blueshifted H2 emission detected from RU Lupi, T
Tau, and DG Tau is consistent with an origin in an outflow. The H2 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 H2 lines from
RU Lupi, DF Tau, and T Tau also have excess blueshifted H2 emission that
extends to as much as -100 km/s. 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 H2 emission by stellar winds
restricts the H2 emission to be formed close to the star. The Ly-alpha and the
H2 emission blueshifted by 15 km/s relative to RU Lupi are extended to the SW
by 0.07 arcsec, although the faster H2 gas that extends to about 100 km/s is
not spatially extended. We also find a small reservoir of H2 emission from TW
Hya and DF Tau consistent with an excitation temperature of about 2.5x10^4 K.Comment: 38 pages, 18 figures, 14 tables. Accepted by ApJ
On the Possible Sources of D/H Dispersion at High Redshift
Recent observations suggest the existence of a white dwarf population in the
Galactic halo, while others suggest that deuterium has been astrated in systems
at high redshift and low metallicity. We propose that these observations could
be signatures of an early population of intermediate-mass stars. Such a
population requires a Population III initial mass function different from that
of the solar neighborhood, as perhaps also suggested by the observed cosmic
infrared background. Also, to avoid overproduction of C and N, it is required
that the Z=0 yields of these stars have low ( solar) abundances
as suggested by some recent calculations. Under these assumptions, we present a
model which reproduces the observed D vs Si trend, and predicts a high cosmic
Type Ia supernova rate, while producing a white dwarf population that accounts
for only 1.5% of the dark halo. This scenario can be tested by
observations of the cosmic supernova rate, and by confirmation and further
studies of the putative white dwarf halo population.Comment: AASTeX, 17 pages, 3 embedded eps fig
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