300 research outputs found
Observations of Small Scale ISM Structure in Dense Atomic Gas
We present high resolution (R~170,000) Kitt Peak National Observatory Co'ude
Feed telescope observations of the interstellar KI 7698 angstrom line towards 5
multiple star systems with saturated NaI components. We compare the KI
absorption line profiles in each of the two (or three) lines of sight in these
systems, and find significant differences between the sight-lines in 3 out of
the 5 cases. We infer that the small scale structure traced by previous NaI
observations is also present in at least some of the components with saturated
NaI absorption lines, and thus the small scale structures traced by the neutral
species are occurring at some level in clouds of all column densities. We
discuss the implications of that conclusion and a potential explanation by
density inhomogeneities
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15
Using the DensePak fiber optic array on the KPNO WIYN telescope, we have
obtained high S/N echelle spectra of the Na I D wavelength region toward the
central 27" x 43" of the globular cluster M15 at a spatial resolution of 4".
The spectra exhibit significant interstellar Na I absorption at LSR velocities
of +3 km/s (LISM component) and +68 km/s (IVC component). Both components vary
appreciably in strength on these scales. The derived Na I column densities
differ by a factor of 4 across the LISM absorption map and by a factor of 16
across the IVC map. Assuming distances of 500 pc and 1500 pc for the LISM and
IVC clouds, these maps show evidence of significant ISM structure down to the
minimum scales of 2000 AU and 6000 AU probed in these absorbers. The
smallest-scale N(Na I) variations observed in the M15 LISM and IVC maps are
typically comparable to or higher than the values found at similar scales in
previous studies of interstellar Na I structure toward binary stars. The
physical implications of the small and larger-scale Na I features observed in
the M15 maps are discussed in terms of variations in the H I column density as
well as in the Na ionization equilibrium.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
The Physical Characteristics of the Small-Scale Interstellar Structure towards Mu Crucis
We present HST/GHRS echelle observations of multiple interstellar lines of
CI, MgI, CrII, and ZnII towards both stars in the mu Cru binary system. Despite
large differences in the profiles of the neutral species, no significant
variations between the stars are seen in the CrII and ZnII line profiles. In
particular, the ZnII absorption observed at -8.6 km/sec towards mu Cru is
constant despite greatly enhanced columns of the neutral species at this
velocity towards mu^1 Cru. An analysis of the fine-structure excitation of CI
in this cloud implies that the density is n_H < 250 cm^{-3}. From the lack of
variation in the (optical) NaI D2 line profiles towards mu^1 and mu^2 Cru in
spectra taken 21 months apart, we can place a lower limit to the size of the
structures of ~10 AU. These results are discussed in the context of recent
radio and optical studies of apparently pervasive high density small-scale
interstellar structure.Comment: 10 pages, 2 figures, to appear in the Astrophysical Journal (Letters
Atomic Diagnostics of X-ray Irradiated Protoplanetary Disks
We study atomic line diagnostics of the inner regions of protoplanetary disks
with our model of X-ray irradiated disk atmospheres which was previously used
to predict observable levels of the NeII and NeIII fine-structure transitions
at 12.81 and 15.55mum. We extend the X-ray ionization theory to sulfur and
calculate the fraction of sulfur in S, S+, S2+ and sulfur molecules. For the
D'Alessio generic T Tauri star disk, we find that the SI fine-structure line at
25.55mum is below the detection level of the Spitzer Infrared Spectrometer
(IRS), in large part due to X-ray ionization of atomic S at the top of the
atmosphere and to its incorporation into molecules close to the mid-plane. We
predict that observable fluxes of the SII 6718/6732AA forbidden transitions are
produced in the upper atmosphere at somewhat shallower depths and smaller radii
than the neon fine-structure lines. This and other forbidden line transitions,
such as the OI 6300/6363AA and the CI 9826/9852AA lines, serve as complementary
diagnostics of X-ray irradiated disk atmospheres. We have also analyzed the
potential role of the low-excitation fine-structure lines of CI, CII, and OI,
which should be observable by SOFIA and Herschel.Comment: Accepted by Ap
Nonlinear Evolution of Very Small Scale Cosmological Baryon Perturbations at Recombination
The evolution of baryon density perturbations on very small scales is
investigated. In particular, the nonlinear growth induced by the radiation drag
force from the shear velocity field on larger scales during the recombination
epoch, which is originally proposed by Shaviv in 1998, is studied in detail. It
is found that inclusion of the diffusion term which Shaviv neglected in his
analysis results in rather mild growth whose growth rate is instead
of enormous amplification of Shaviv's original claim since the
diffusion suppresses the growth. The growth factor strongly depends on the
amplitude of the large scale velocity field. The nonlinear growth mechanism is
applied to density perturbations of general adiabatic cold dark matter (CDM)
models. In these models, it has been found in the previous works that the
baryon density perturbations are not completely erased by diffusion damping if
there exists gravitational potential of CDM. With employing the perturbed rate
equation which is derived in this paper, the nonlinear evolution of baryon
density perturbations is investigated. It is found that: (1) The nonlinear
growth is larger for smaller scales. This mechanism only affects the
perturbations whose scales are smaller than , which are
coincident with the stellar scales. (2) The maximum growth factors of baryon
density fluctuations for various COBE normalized CDM models are typically less
than factor 10 for large scale velocity peaks. (3) The growth factor
depends on .Comment: 24 pages, 9 figures, submitted to Ap
The Crab Nebula's Composition and Precursor Star Mass
We present results of new photoionization calculations for investigating
gaseous regions that represent potentially expected stages of nuclear
processing in the Crab Nebula supernova remnant. In addition to gas resulting
from CNO-processing and oxygen-burning, as previously reported, a large
component of the nebula appears to be carbon-rich. These results suggest that
the precursor star had an initial mass of 9.5 solar masses or more.Comment: manuscript in AASTeX, 6 figures in .eps, submitted to Astronomical
Journa
Ultrafast effective multi-level atom method for primordial hydrogen recombination
Cosmological hydrogen recombination has recently been the subject of renewed
attention because of its importance for predicting the power spectrum of cosmic
microwave background anisotropies. It has become clear that it is necessary to
account for a large number n >~ 100 of energy shells of the hydrogen atom,
separately following the angular momentum substates in order to obtain
sufficiently accurate recombination histories. However, the multi-level atom
codes that follow the populations of all these levels are computationally
expensive, limiting recent analyses to only a few points in parameter space. In
this paper, we present a new method for solving the multi-level atom
recombination problem, which splits the problem into a computationally
expensive atomic physics component that is independent of the cosmology, and an
ultrafast cosmological evolution component. The atomic physics component
follows the network of bound-bound and bound-free transitions among excited
states and computes the resulting effective transition rates for the small set
of "interface" states radiatively connected to the ground state. The
cosmological evolution component only follows the populations of the interface
states. By pre-tabulating the effective rates, we can reduce the recurring cost
of multi-level atom calculations by more than 5 orders of magnitude. The
resulting code is fast enough for inclusion in Markov Chain Monte Carlo
parameter estimation algorithms. It does not yet include the radiative transfer
or high-n two-photon processes considered in some recent papers. Further work
on analytic treatments for these effects will be required in order to produce a
recombination code usable for Planck data analysis.Comment: Version accepted by Phys. Rev. D. Proof of equivalence of effective
and standard MLA methods moved to the main text. Some rewording
Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
We discuss the results of continued spectroscopic monitoring of the variable
intermediate-velocity (IV) absorption at v = -38 km/s toward HD 219188. After
reaching maxima in mid-2000, the column densities of both Na I and Ca II in
that IV component declined by factors >= 2 by the end of 2006. Comparisons
between HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS
echelle spectra obtained in 1994--1995 indicate the following: (1) The
absorption from the dominant species S II, O I, Si II, and Fe II is roughly
constant in all four sets of spectra -- suggesting that the total N(H) and the
(mild) depletions have not changed significantly over a period of nearly ten
years. (2) The column densities of the trace species C I (both ground and
excited fine-structure states) and of the excited state C II* all increased by
factors of 2--5 between 1995 and 2001 -- implying increases in the hydrogen
density n_H (from about 20 cm^{-3} to about 45 cm^{-3}) and in the electron
density n_e (by a factor >= 3) over that 6-year period. (3) The column
densities of C I and C II* -- and the corresponding inferred n_H and n_e --
then decreased slightly between 2001 and 2004. (4) The changes in C I and C II*
are very similar to those seen for Na I and Ca II. The relatively low total
N(H) and the modest n_H suggest that the -38 km/s cloud toward HD 219188 is not
a very dense knot or filament. Partial ionization of hydrogen appears to be
responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this
case, the variations in those species appear to reflect differences in density
and ionization [and not N(H)] over scales of tens of AU.Comment: 33 pages, 6 figures, aastex, accepted to Ap
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