140 research outputs found
Three-dimensional chemically homogeneous and bi-abundance photoionization models of the "super-metal-rich" planetary nebula NGC 6153
Deep spectroscopy of the planetary nebula (PN) NGC\,6153 shows that its heavy
element abundances derived from optical recombination lines (ORLs) are ten
times higher than those derived from collisionally excited lines (CELs), and
points to the existence of H-deficient inclusions embedded in the diffuse
nebula. In this study, we have constructed chemically homogeneous and
bi-abundance three-dimensional photoionization models, using the Monte Carlo
photoionization code {\sc mocassin}. We attempt to reproduce the multi-waveband
spectroscopic and imaging observations of NGC\,6153, and investigate the nature
and origin of the postulated H-deficient inclusions, as well as their impacts
on the empirical nebular analyses assuming a uniform chemical composition. Our
results show that chemically homogeneous models yield small electron
temperature fluctuations and fail to reproduce the strengths of ORLs from C, N,
O and Ne ions. In contrast, bi-abundance models incorporating a small amount of
metal-rich inclusions ( per cent of the total nebular mass) are able
to match all the observations within the measurement uncertainties. The
metal-rich clumps, cooled down to a very low temperature (~K) by
ionic infrared fine-structure lines, dominate the emission of heavy element
ORLs, but contribute almost nil to the emission of most CELs. We find that the
abundances of C, N, O and Ne derived empirically from CELs, assuming a uniform
chemical composition, are about 30 per cent lower than the corresponding
average values of the whole nebula, including the contribution from the
H-deficient inclusions. Ironically, in the presence of H-deficient inclusions,
the traditional standard analysis of the optical helium recombination lines,
assuming a chemically homogeneous nebula, overestimates the helium abundance by
40 per cent.Comment: 19 pages, 18 figures, accepted for publication in MNRA
A near-solar metallicity damped Lyman-alpha system toward the BAL quasar Tol 1037-2703
We report the detection of a Broad Absorption Line (BAL) outflow in the
spectrum of Tol 1037-2703 with three main BALs at 36000, 25300 and 22300~\kms
outflow velocities. Although the overall flow is dominated by high ionization
lines like and C IV, the gas of highest velocity shows absorption from Mg I, Mg
II and Fe II. Covering factor arguments suggest that the absorbing complexes
are physically associated with the QSO and have transverse dimensions smaller
than that of the UV continuum emitting region (r<0.1 pc). We show that the C IV
absorption at \zabs=2.082 has a covering factor 0.86 and the absorption
profile has varied over the last four years. The detection of absorption from
excited fine structure levels of C II and Si II in narrow components embedded
in the C IV trough reveals large density inhomogeneities. IR pumping is the
most likely excitation process. The \zabs=2.139 system is a damped system with
log N(H I) ~ 19.7. The weakness of the metal lines together with the high
quality of the data make the metallicity measurements particularly reliable.
The absolute metallicity is close to solar with [Zn/H]=-0.26. The
-chain elements have metallicities consistently solar (respectively
+0.05, -0.02, -0.03 and -0.15 for [Mg/H], [Si/H], [P/H] and [S/H]) and iron
peak elements are depleted by a factor of about two ([Fe/Zn], [Cr/Zn], [Mn/Zn]
and [Ni/Zn] are equal to -0.39, -0.27, -0.49, -0.30). Lines from C I are
detected but H is absent with a molecular fraction less than
8. High metallicity and low nitrogen abundance, [N/Zn]=-1.40,
favor the idea that metals have been released by massive stars during a
starburst of less than 0.5 Gyr of age. (abridged)Comment: To appear in A & A, 12 pages, latex, 15 figure
Rubidium in the Interstellar Medium
We present observations of interstellar rubidium toward o Per, zeta Per, AE
Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable 85Rb
and long-lived 87Rb are produced predominantly by high-mass stars, through a
combination of the weak s- and r-processes. The 85Rb/87Rb ratio was determined
from measurements of the Rb I line at 7800 angstroms and was compared to the
solar system meteoritic ratio of 2.59. Within 1-sigma uncertainties all
directions except HD 147889 have Rb isotope ratios consistent with the solar
system value. The ratio toward HD 147889 is much lower than the meteoritic
value and similar to that toward rho Oph A (Federman et al. 2004); both lines
of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was
attributed to a deficit of r-processed 85Rb. Our larger sample suggests instead
that 87Rb is enhanced in these two lines of sight. When the total elemental
abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K
ratio is significantly lower than the meteoritic ratio for all the sight lines
in this study. Available interstellar samples for other s- and r- process
elements are used to help interpret these results.Comment: 24 pages, 3 figures. Accepted for publication in Ap
Influence of primordial magnetic fields on 21 cm emission
Magnetic fields in the early universe can significantly alter the thermal
evolution and the ionization history during the dark ages. This is reflected in
the 21 cm line of atomic hydrogen, which is coupled to the gas temperature
through collisions at high redshifts, and through the Wouthuysen-Field effect
at low redshifts. We present a semi-analytic model for star formation and the
build-up of a Lyman alpha background in the presence of magnetic fields, and
calculate the evolution of the mean 21 cm brightness temperature and its
frequency gradient as a function of redshift. We further discuss the evolution
of linear fluctuations in temperature and ionization in the presence of
magnetic fields and calculate the effect on the 21 cm power spectrum. At high
redshifts, the signal is increased compared to the non-magnetic case due to the
additional heat input into the IGM from ambipolar diffusion and the decay of
MHD turbulence. At lower redshifts, the formation of luminous objects and the
build-up of a Lyman alpha background can be delayed by a redshift interval of
10 due to the strong increase of the filtering mass scale in the presence of
magnetic fields. This tends to decrease the 21 cm signal compared to the
zero-field case. In summary, we find that 21 cm observations may become a
promising tool to constrain primordial magnetic fields.Comment: 14 pages, 11 figures, accepted for publication at Ap
Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the \ion{He}{1} recombination line ratios,
designated (\ion{He}{1}), are presented for 48 planetary nebulae
(PNe). We study the effect that temperature fluctuations inside nebulae have on
the (\ion{He}{1}) value. We show that a comparison between (\ion{He}{1}) and the electron temperature derived from the Balmer jump of
the \ion{H}{1} recombination spectrum, designated (\ion{H}{1}),
provides an opportunity to discriminate between the paradigms of a chemically
homogeneous plasma with temperature and density variations, and a two-abundance
nebular model with hydrogen-deficient material embedded in diffuse gas of a
``normal'' chemical composition (i.e. solar), as the possible causes of
the dichotomy between the abundances that are deduced from collisionally
excited lines to those deduced from recombination lines. We find that (\ion{He}{1}) values are significantly lower than (\ion{H}{1})
values, with an average difference of (\ion{H}{1})-(\ion{He}{1}) K. The result is consistent with the expectation of
the two-abundance nebular model but is opposite to the prediction of the
scenarios of temperature fluctuations and/or density inhomogeneities. From the
observed difference between (\ion{He}{1}) and (\ion{H}{1}), we estimate that the filling factor ofhydrogen-deficient
components has a typical value of . In spite of its small mass, the
existence of hydrogen-deficient inclusions may potentially have a profound
effect in enhancing the intensities of \ion{He}{1} recombination lines and
thereby lead to apparently overestimated helium abundances for PNe.Comment: 27 pages, 7 figures, accepted for publication in MNRA
CMB constraints on the fine structure constant
We study constraints on time variation of the fine structure constant alpha
from cosmic microwave background (CMB) taking into account simultaneous change
in alpha and the electron mass m_e which might be implied in unification
theories. We obtain the constraints -0.097 < Delta alpha/alpha < 0.034 at 95%
C.L. using WMAP data only, and -0.042 < Delta alpha/alpha < 0.026 combining
with the constraint on the Hubble parameter by the HST Hubble Key Project.
These are improved by 15% compared with constraints assuming only alpha varies.
We discuss other relations between variations in alpha and m_e but we do not
find evidence for varying alpha.Comment: 19 pages, 8 figure
Slit Observations and Empirical Calculations for HII Regions
When analysing HII regions, a possible source of systematic error on
empirically derived physical quantities is the limited size of the slit used
for the observations. A grid of photoionization models was built through the
Aangaba code varying the ionizing radiation spectrum emitted by a stellar
cluster, as well as the gas abundance. The calculated line surface brightness
was then used to simulate slit observations and to derive empirical parameters
using the usual methods described in the literature. Depending on the fraction
of the object covered by the slit, the parameters can be different from those
obtained from observations of the whole object, an effect that is mainly
dependent on the age of the ionizing stellar cluster. The low-ionization
forbidden lines are more sensitive to the size of the area covered by the slit
than the high-ionization forbidden lines or recombination lines. Regarding the
temperature indicator T[OIII], the slit effects are small since this
temperature is derived from [OIII] lines. On the other hand, for the abundance
indicator R23, which depends also on the [OII] line, the slit effect is
slightly higher. Therefore, the systematic error due to slit observations on
the O abundance is low, being usually less than 10%, except for HII regions
powered by stellar clusters with a relative low number of ionizing photons
between 13.6 and 54.4 eV, which create a smaller O++ emitting volume. In this
case, the systematic error on the empirical O abundance deduced from slit
observations is more than 10% when the covered area is less than 50%.Comment: To be published in MNRAS, accepted in 09/09/2005, 17 pages and 6
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Detection of [O I] 63 <i>μ</i>m in absorption toward Sgr B2
A high signal-to-noise 52-90 μm spectrum is presented for the central part of the Sagittarius B2 complex. The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The [O I] 63 μm line is detected in absorption even at the grating spectral resolution of 0.29 μm. A lower limit for the column density of atomic oxygen of the order of 1019 cm-2 is derived. This implies that more than 40% of the interstellar oxygen must be in atomic form along the line of sight toward the Sgr B2 molecular cloud
Unveiling the structure of the planetary nebula M 2-48: Kinematics and physical conditions
The kinematics and physical conditions of the bipolar planetary nebula M 2-48
are analysed from high and low dispersion long-slit spectra. Previous CCD
narrow-band optical observations have suggested that this nebula is mainly
formed by a pair of symmetric bow-shocks, an off-center semi-circular shell,
and an internal bipolar structure. The bipolar outflow has a complex structure,
characterised by a series of shocked regions located between the bright core
and the polar tips. There is an apparent kinematic discontinuity between the
bright bipolar core and the outer regions. The fragmented ring around the
bright bipolar region presents a low expansion velocity and could be associated
to ejection in the AGB-PN transition phase, although its nature remains
unclear. The chemical abundances of the central region are derived, showing
that M 2-48 is a Type I planetary nebula (PN)
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