276 research outputs found
Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward delta Orionis by the Interstellar Medium Absorption Profile Spectrograph
Studies of the abundances of deuterium in different astrophysical sites are
of fundamental importance to answering the question about how much deuterium
was produced during big bang nucleosynthesis and what fraction of it was
destroyed later. With this in mind, we used the Interstellar Medium Absorption
Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a
wavelength resolution of 4 km/s (FWHM) the L-delta and L-epsilon absorption
features produced by interstellar atomic deuterium in the spectrum of delta Ori
A. A chi-square analysis indicated that 0.96 < N(D I)< 1.45e15 cm^{-2} at a 90%
level of confidence, and the gas is at a temperature of about 6000K. To obtain
an accurate value of N(H I) needed for a determination of the atomic ratio of D
to H, we measured the L-alpha absorption features in 57 spectra of delta Ori in
the IUE archive. From our measurement of N(H I)= 1.56e20 cm^{-2}, we found that
N(D I)/N(H I)= 7.4(+1.9,-1.3)e-6 (90% confidence). Our result for D/H contrasts
with the more general finding along other lines of sight that D/H is
approximately 1.5e-5. The underabundance of D toward delta Ori A is not
accompanied by an overabundance of N or O relative to H, as one might expect if
the gas were subjected to more stellar processing than usual.Comment: 37 pages, 6 figures. Submitted to the Astrophysical Journa
What is the Total Deuterium Abundance in the Local Galactic Disk?
Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer
(FUSE) satellite, together with spectra from the Copernicus and IMAPS
instruments, reveal an unexplained very wide range in the observed
deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk
beyond the Local Bubble. We argue that spatial variations in the depletion of
deuterium onto dust grains can explain these local variations in the observed
gas-phase D/H ratios. We present a variable deuterium depletion model that
naturally explains the constant measured values of D/H inside the Local Bubble,
the wide range of gas-phase D/H ratios observed in the intermediate regime (log
N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger
hydrogen column densities. We consider empirical tests of the deuterium
depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions
of the refractory metals iron and silicon, and (ii) correlation with the
molecular hydrogen rotational temperature. Both of these tests are consistent
with deuterium depletion from the gas phase in cold, not recently shocked,
regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked
or otherwise heated recently. We argue that the most representative value for
the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4
(1 sigma) parts per million (ppm). This ratio constrains Galactic chemical
evolution models to have a very small deuterium astration factor, the ratio of
primordial to total (D/H) ratio in the local region of the Galactic disk, which
we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma)
depending on the adopted light element nuclear reaction rates.Comment: 19 pages, 9 figure
The evolution of ultraviolet emission lines from the circumstellar material surrounding SN 1987A
The presence of narrow high-temperature emission lines from nitrogen-rich gas
close to SN 1987A has been the principal observational constraint on the evolu-
tionary status of the supernova's progenitor. A new analysis of the complete
five-year set of low and high resolution IUE ultraviolet spectra of SN 1987A
(1987.2--1992.3) provide fluxes for the N V 1240, N IV] 1486, He II 1640, OIII]
1665, NIII] 1751, and CIII] 1908 lines with significantly reduced random and
systematic errors and reveals significant short-term fluctuations in the light
curves. The N V, N IV] and N III] lines turn on sequentially over 15 to 20 days
and show a progression from high to low ionization potential, implying an ioni-
zation gradient in the emitting region. The line emission turns on suddenly at
83+/-4 days after the explosion, as defined by N IV]. The N III] line reaches
peak luminosity at 399+/-15 days. A ring radius of (6.24+/-0.20)E{17} cm and
inclination of 41.0+/-3.9 is derived from these times, assuming a circular
ring. The probable role of resonant scattering in the N V light curve
introduces systematic errors that leads us to exclude this line from the timing
analysis. A new nebular analysis yields improved CNO abundance ratios
N/C=6.1+/-1.1 and N/O=1.7+/-0.5, confirming the nitrogen enrichment found in
our previous paper. From the late-time behavior of the light curves we find
that the emission origi- nates from progressively lower density gas. We
estimate the emitting mass near maximum (roughly 400 days) to be roughly
4.7E{-2} solar masses, assuming a filling factor of unity and an electron
density of 2.6E4 cm^{-3}. These results are discussed in the context of current
models for the emission and hydrodynamics of the ring.Comment: 38 pages, AASTeX v.4.0, 13 Postscript figures; ApJ, in pres
Distribution and Kinematics of O VI in the Galactic Halo
FUSE spectra of 100 extragalactic objects are analyzed to obtain measures of
O VI absorption along paths through the Milky Way thick disk/halo. Strong O VI
absorption over the approximate velocity range from -100 to 100 km/s reveals a
widespread but highly irregular distribution of thick disk O VI, implying the
existence of substantial amounts of hot gas with T ~ 3x10^5 K in the Milky Way
halo. Large irregularities in the distribution of the absorbing gas are found
to be similar over angular scales extending from less than one to 180 degrees,
indicating a considerable amount of small and large scale structure in the gas.
The overall distribution of Galactic O VI is not well described by a
symmetrical plane-parallel layer of patchy O VI absorption. The simplest
departure from such a model that provides a reasonable fit to the observations
is a plane-parallel patchy absorbing layer with a scale height of 2.3 kpc, and
a 0.25 dex excess of O VI in the northern Galactic polar region. The O VI
absorption has a Doppler parameter b = 30 to 99 km/s, with an average value of
60 km/s . Thermal broadening alone cannot explain the large observed profile
widths. The average O VI absorption velocities toward high latitude objects
range from -46 to 82 km/s, with a sample average of 0 km/s and a standard
deviation of 21 km/s. O VI associated with the thick disk moves both toward and
away from the plane with roughly equal frequency. A combination of models
involving the radiative cooling of hot fountain gas, the cooling of supernova
bubbles in the halo, and the turbulent mixing of warm and hot halo gases is
required to explain the presence of O VI and other highly ionized atoms found
in the halo. (abbreviated)Comment: 70 pages, single-spaced, PDF format. Bound copies of this manuscript
and two accompanying articles are available upon request. Submitted to ApJ
A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds
We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to
the Small and Large Magellanic Clouds, using hot stars as background sources.
FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of
SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912
and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column
densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced
H2 abundances in the Magellanic Clouds relative to the Milky Way, with average
molecular fractions = 0.010 (+0.005, -0.002) for the SMC and =
0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the
Galactic disk over a similar range of reddening. The dominant uncertainty in
this measurement results from the systematic differences between 21 cm radio
emission and Lya in pencil-beam sight lines as measures of N(HI). These results
imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x
10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm
emission measurements. The LMC and SMC abundance patterns can be reproduced in
ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x
10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the
Galactic mean value. We find that these high-radiation, low-formation-rate
models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational
excitation ratios in the Clouds. We use H2 column densities in low rotational
states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature
= 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic
gas. We discuss the implications of this work for theories of star formation in
low-metallicity environments. [Abstract abridged]Comment: 30 pages emulateapj, 14 figures (7 color), 7 tables, accepted for
publication in the Astrophysical Journal, figures 11 and 12 compressed at
slight loss of quality, see http://casa.colorado.edu/~tumlinso/h2/ for full
version
Carbon Monoxide in the Cold Debris of Supernova 1987A
We report spectroscopic and imaging observations of rotational transitions of
cold CO and SiO in the ejecta of SN1987A, the first such emission detected in a
supernova remnant. In addition to line luminosities for the CO J=1-0, 2-1, 6-5,
and 7-6 transitions, we present upper limits for all other transitions up to
J=13-12, collectively measured from the Atacama Large Millimeter Array (ALMA),
the Atacama Pathfinder EXperiment (APEX), and the Herschel Spectral and
Photometric Imaging REceiver (SPIRE). Simple models show the lines are emitted
from at least 0.01 solar masses of CO at a temperature > 14 K, confined within
at most 35% of a spherical volume expanding at ~ 2000 km/s. Moreover, we locate
the emission within 1'' of the central debris. These observations, along with a
partial observation of SiO, confirm the presence of cold molecular gas within
supernova remnants and provide insight into the physical conditions and
chemical processes in the ejecta. Furthermore, we demonstrate the powerful new
window into supernova ejecta offered by submillimeter observations.Comment: Accepted to the Astrophysical Journal Letters, 6 pages, 3 figure
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
Preliminary Spectral Analysis of the Type II Supernova 1999em
We have calculated fast direct spectral model fits to two early-time spectra
of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The
first is an extremely early blue optical spectrum and the second a combined HST
and optical spectrum obtained one week later. Spectroscopically this supernova
appears to be a normal Type II and these fits are in excellent agreement with
the observed spectra. Our direct analysis suggests the presence of enhanced
nitrogen. We have further studied these spectra with the full NLTE general
model atmosphere code PHOENIX. While we do not find confirmation for enhanced
nitrogen (nor do we rule it out), we do require enhanced helium. An even more
intriguing possible line identification is complicated Balmer and He I lines,
which we show falls naturally out of the detailed calculations with a shallow
density gradient. We also show that very early spectra such as those presented
here combined with sophisticated spectral modeling allows an independent
estimate of the total reddening to the supernova, since when the spectrum is
very blue, dereddening leads to changes in the blue flux that cannot be
reproduced by altering the ``temperature'' of the emitted radiation. These
results are extremely encouraging since they imply that detailed modeling of
early spectra can shed light on both the abundances and total extinction of SNe
II, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54
Modeling of the Super-Eddington Phase for Classical Novae: Five IUE Novae
We present a light curve model for the super-Eddington luminosity phase of
five classical novae observed with IUE. Optical and UV light curves are
calculated based on the optically thick wind theory with a reduced effective
opacity for a porous atmosphere. Fitting a model light curve with the UV 1455
\AA light curve, we determine the white dwarf mass and distance to be (1.3
M_sun, 4.4 kpc) for V693 CrA, (1.05 M_sun, 1.8 kpc) for V1974 Cyg, (0.95 M_sun,
4.1 kpc) for V1668 Cyg, (1.0 M_sun, 2.1 kpc) for V351 Pup, and (1.0 M_sun, 4.3
kpc) for OS And.Comment: 9 pages including 8 figures, to appear in the Astrophysical Journa
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