48 research outputs found
High-Resolution Keck Spectra of the Associated Absorption Lines in 3C 191
Associated absorption lines (AALs) are valuable probes of the gaseous
environments near quasars. Here we discuss high-resolution (6.7 km/s) spectra
of the AALs in the radio-loud quasar 3C 191 (redshift z=1.956). The measured
AALs have ionizations ranging from Mg I to N V, and multi-component profiles
that are blueshifted by ~400 to ~1400 km/s relative to the quasar's broad
emission lines. These data yield the following new results. 1) The density
based on Si II*/Si II lines is ~300 cm-3, implying a distance of ~28 kpc from
the quasar if the gas is photoionized. 2) The characteristic flow time is thus
\~3 x 10^7 yr. 3) Strong Mg I AALs identify neutral gas with very low
ionization parameter and high density. We estimate n_H > 5 x 10^4 cm-3 in this
region, compared to ~15 cm-3 where the N V lines form. 4) The total column
density is N_H < 4 x 10^18 cm-2 in the neutral gas and N_H ~ 2 x 10^20 cm-2 in
the moderately ionized regions. 5) The total mass in the AAL outflow is M ~ 2 x
10^9 Mo, assuming a global covering factor (as viewed from the quasar) of ~10%
>. 6) The absorbing gas only partially covers the background light source(s)
along our line(s) of sight, requiring absorption in small clouds or filaments
<0.01 pc across. The ratio N_H/n_H implies that the clouds have radial (line-
of-sight) thicknesses <0.2 pc. These properties might characterize a sub-class
of AALs that are physically related to quasars but form at large distances. We
propose a model for the absorber in which pockets of dense neutral gas are
surrounded by larger clouds of generally lower density and higher ionization.
This outflowing material might be leftover from a blowout associated with a
nuclear starburst, the onset of quasar activity or a past broad absorption line
(BAL) wind phase.Comment: 15 pages text plus 6 figures, in press with Ap
ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations
We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC
6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S
III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine
ionic sulfur abundances in these H II regions. There is relatively good
agreement between our observations and predictions of S^+3 abundances based on
photoionization calculations, although there is an offset in the sense that the
models overpredict the S^+3 abundances. We emphasize a need for more
observations of this type in order to place nebular sulfur abundance
determinations on firmer ground. The S/O ratios derived using the ISO
observations in combination with optical data are consistent with values of
S/O, derived from optical measurements of other metal-poor galaxies.
We present a new formalism for the simultaneous determination of the
temperature, temperature fluctuations, and abundances in a nebula, given a mix
of optical and infrared observed line ratios. The uncertainties in our ISO
measurements and the lack of observations of [S III] lambda 9532 or lambda 9069
do not allow an accurate determination of the amplitude of temperature
fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we
illustrate the diagnostic power and limitations of our new method.Comment: 32 Pages total, including 6 encapsulated postscript figures (one with
two parts). Accepted for Publication in the 20 Dec 2002 Ap
Morphology and Composition of the Helix Nebula
We present new narrow-band filter imagery in H-alpha and [N II] 6584 along
with UV and optical spectrophotometry measurements from 1200 to 9600 Angstroms
of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large
diameter and low surface brightness. Detailed models of the observable ionized
nebula support the recent claim that the Helix is actually a flattened disk
whose thickness is roughly one-third its diameter with an inner region
containing hot, highly ionized gas which is generally invisible in narrow-band
images. The outer visible ring structure is of lower ionization and temperature
and is brighter because of a thickening in the disk. We also confirm a central
star effective temperature and luminosity of 120,000K and 100L_sun, and we
estimate a lower limit to the nebular mass to be 0.30M_sun. Abundance
measurements indicate the following values: He/H=0.12 (+/-0.017),
O/H=4.60x10^-4 (+/-0.18), C/O=0.87 (+/-0.12), N/O=0.54 (+/-0.14), Ne/O=0.33
(+/-0.04), S/O=3.22x10^-3 (+/-0.26), and Ar/O=6.74x10^-3 (+/-0.76). Our carbon
abundance measurements represent the first of their kind for the Helix Nebula.
The S/O ratio which we derive is anomalously low; such values are found in only
a few other planetary nebulae. The central star properties, the super-solar
values of He/H and N/O, and a solar level of C/O are consistent with a 6.5M_sun
progenitor which underwent three phases of dredge-up and hot bottom burning
before forming the planetary nebula.Comment: 50-page manuscript plus 11 postscript figures. This revised version
corrects a typo in earlier submission. Nothing else has changed. Accepted for
publication in the Astrophysical Journa
Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum
The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of
the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a
compilation of UV to IR narrow emission line data to determine the spectral
energy distribution (SED) of the obscured extreme-UV continuum that
photoionizes the narrow line emitting gas in the active galactic nucleus. We
search a large grid of gas cloud models and SEDs for the combination that best
reproduces the observed line fluxes and NLR geometry. Our best fit model
reproduces the observed line fluxes to better than a factor of 2 on average and
is in general agreement with the observed NLR geometry. It has two gas
components that are consistent with a clumpy distribution of dense outflowing
gas in the center and a more extended distribution of less dense and more
clumpy gas farther out that has no net outflow. The best fit SED has a deep
trough at ~4 Ryd, which is consistent with an intrinsic Big Blue Bump that is
partially absorbed by ~6x10^19 cm^-2 of neutral hydrogen interior to the NLR.Comment: 15 pp, 4 figures, ApJ accepte
Sulfur, Chlorine, & Argon Abundances in Planetary Nebulae. I: Observations and Abundances in a Northern Sample
This paper is the first of a series specifically studying the abundances of
sulfur, chlorine, and argon in Type II planetary nebulae (PNe) in the Galactic
disk. Ratios of S/O, Cl/O, and Ar/O constitute important tests of differential
nucleosynthesis of these elements and serve as strict constraints on massive
star yield predictions. We present new ground-based optical spectra extending
from 3600-9600 Angstroms for a sample of 19 Type II northern PNe. This range
includes the strong near infrared lines of [S III] 9069,9532, which allows us
to test extensively their effectiveness as sulfur abundance indicators. We also
introduce a new, model-tested ionization correction factor for sulfur. For the
present sample, we find average values of S/O=1.2E-2(+/- 0.71E-2),
Cl/O=3.3E-4(+/- 1.6E-4), and Ar/O=5.0E-3(+/- 1.9E-3).Comment: 44 pages, 6 figures. Accepted for publication in the Astrophysical
Journa
On the Dynamical Overstability of Radiative Blast Waves: The Atomic Physics of Shock Stability
Atomic physics calculations of radiative cooling are used to calculate
criteria for the overstability of radiating shocks. Our calculations explain
the measurement of shock overstability by Grun et al. and explain why the
overstability was not observed in other experiments. The methodology described
here can be especially useful in astrophysical situations where the relevant
properties leading to an overstability can be measured spectroscopically, but
the effective adiabatic index is harder to determine.Comment: 11 pages including 3 figures, accepted for publication in Physical
Review Letter
The coronal line regions of planetary nebulae NGC6302 and NGC6537: 3-13um grating and echelle spectroscopy
We report on advances in the study of the cores of NGC6302 and NGC6537 using
infrared grating and echelle spectroscopy. In NGC6302, emission lines from
species spanning a large range of ionization potential, and in particular
[SiIX]3.934um, are interpreted using photoionization models (including CLOUDY),
which allow us to reestimate the central star's temperature to be about
250000K. All of the detected lines are consistent with this value, except for
[AlV] and [AlVI]. Aluminium is found to be depleted to one hundredth of the
solar abundance, which provides further evidence for some dust being mixed with
the highly ionized gas (with photons harder than 154eV). A similar depletion
pattern is observed in NGC6537. Echelle spectroscopy of IR coronal ions in
NGC6302 reveals a stratified structure in ionization potential, which confirms
photoionization to be the dominant ionization mechanism. The lines are narrow
(< 22km/s FWHM), with no evidence of the broad wings found in optical lines
from species with similar ionization potentials, such as [NeV]3426A. We note
the absence of a hot bubble, or a wind blown bipolar cavity filled with a hot
plasma, at least on 1'' and 10km/s scales. We also provide accurate new
wavelengths for several of the infrared coronal lines observed with the
echelle.Comment: Accepted for publication in MNRA
CHIANTI - an Atomic Database for Emission Lines. Paper VI: Proton Rates and Other Improvements
The CHIANTI atomic database contains atomic energy levels, wavelengths,
radiative transition probabilities and electron excitation data for a large
number of ions of astrophysical interest. Version 4 has been released, and
proton excitation data is now included, principally for ground configuration
levels that are close in energy. The fitting procedure for excitation data,
both electrons and protons, has been extended to allow 9 point spline fits in
addition to the previous 5 point spline fits. This allows higher quality fits
to data from close-coupling calculations where resonances can lead to
significant structure in the Maxwellian-averaged collision strengths. The
effects of photoexcitation and stimulated emission by a blackbody radiation
field in a spherical geometry on the level balance equations of the CHIANTI
ions can now be studied following modifications to the CHIANTI software. With
the addition of H I, He I and N I, the first neutral species have been added to
CHIANTI. Many updates to existing ion data-sets are described, while several
new ions have been added to the database, including Ar IV, Fe VI and Ni XXI.
The two-photon continuum is now included in the spectral synthesis routines,
and a new code for calculating the relativistic free-free continuum has been
added. The treatment of the free-bound continuum has also been updated.Comment: CHIANTI is available at http://wwwsolar.nrl.navy.mil/chianti.htm
Planetary nebulae in M33: probes of AGB nucleosynthesis and ISM abundances
We have obtained deep optical spectrophotometry of 16 planetary nebulae in
M33, mostly located in the central two kpc of the galaxy, with the Subaru and
Keck telescopes. We have derived electron temperatures and chemical abundances
from the detection of the [OIII]4363 line for the whole sample. We have found
one object with an extreme nitrogen abundance, 12+log(N/H)=9.20, accompanied by
a large helium content. After combining our data with those available in the
literature for PNe and HII regions, we have examined the behavior of nitrogen,
neon, oxygen and argon in relation to each other, and as a function of
galactocentric distance. We confirm the good correlation between Ne/H and O/H
for PNe in M33. Ar/H is also found to correlate with O/H. This strengthens the
idea that at the metallicity of the bright PNe analyzed in M33, which is
similar to that found in the LMC, these elements have not been significantly
modified during the dredge-up processes that take place during the AGB phase of
their progenitor stars. We find no significant oxygen abundance offset between
PNe and HII regions at any given galactocentric distance, despite the fact that
these objects represent different age groups in the evolution of the galaxy.
Combining the results from PNe and HII regions, we obtain a representative
slope of the ISM alpha-element (O, Ar, Ne) abundance gradient in M33 of -0.025
+/- 0.006 dex/kpc. Both PNe and HII regions display a large abundance
dispersion at any given distance from the galactic center. We find that the N/O
ratio in PNe is enhanced, relative to the HII regions, by approximately 0.8
dex.Comment: 21 pages, 20 figures. Accepted for publication in MNRA
Quantitative analysis of WC stars: Constraints on neon abundances from ISO/SWS spectroscopy
Neon abundances are derived in four Galactic WC stars -- gamma Vel (WR11,
WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8), and WR146 (WC5+O8) -
using mid-infrared fine structure lines obtained with ISO/SWS. Stellar
parameters for each star are derived using a non-LTE model atmospheric code
(Hillier & Miller 1998) together with ultraviolet (IUE), optical (INT, AAT) and
infrared (UKIRT, ISO) spectroscopy. In the case of gamma Vel, we adopt results
from De Marco et al. (2000), who followed an identical approach.
ISO/SWS datasets reveal the [NeIII] 15.5um line in each of our targets, while
[NeII] 12.8um, [SIV] 10.5um and [SIII] 18.7um are observed solely in gamma Vel.
Using a method updated from Barlow et al. (1988) to account for clumped winds,
we derive Ne/He=3-4x10^-3 by number, plus S/He=6x10^-5 for gamma Vel. Neon is
highly enriched, such that Ne/S in gamma Vel is eight times higher than cosmic
values. However, observed Ne/He ratios are a factor of two times lower than
predictions of current evolutionary models of massive stars. An imprecise
mass-loss and distance were responsible for the much greater discrepancy in
neon content identified by Barlow et al.
Our sample of WC5--8 stars span a narrow range in T* (=55--71kK), with no
trend towards higher temperature at earlier spectral type, supporting earlier
results for a larger sample by Koesterke & Hamann (1995). Stellar luminosities
range from 100,000 to 500,000 Lo, while 10^-5.1 < Mdot/(Mo/yr) < 10^-4.5,
adopting clumped winds, in which volume filling factors are 10%. In all cases,
wind performance numbers are less than 10, significantly lower than recent
estimates. Carbon abundances span 0.08 < C/He < 0.25 by number, while oxygen
abundances remain poorly constrained.Comment: 16 pages,7 figures accepted for MNRA