126 research outputs found
Endogenous oxygen in the extremely metal-poor planetary nebula PN G135.9+55.9
It is shown that, in contrast to recent claims, oxygen (and helium) may not be extraordinarily underabundant in the new galactic halo planetary nebula (GHPN) PN G135.9+55.9 (hereafter PN G135). Determining elemental abundances in hot, highly ionized objects such as PN G135 depends critically on a proper description of the collisional excitation of the hydrogen Balmer lines, the departure from Case B recombination of hydrogen, the underlying stellar absorption lines, the shape of the primary continuum and the ionization equilibrium of highly ionized species of both oxygen and neon. Conversely, PN G135 provides unique checks of atomic data in unusual conditions: the H I collision strengths obtained by Aggarwal et al. (1991) for 1s - n transitions (3 †n †5) are too large, while those obtained by Anderson et al. (2002) are acceptable. Empirical collision strengths are presented for n > 5. Photoionization models of PN G135 that fit all available optical data can be demonstrated only for oxygen abundances 12 + log (O/H) > 7.2 (>1/30 solar) and values 0.6 dex larger are possible, depending on the assumed C/O abundance ratio. Plausible variations in the geometry of the nebula, the primary stellar continuum and the atomic data do not alter this conclusion. The C/O ratio is less than 10 by number and Ne/O is at most solar. A satisfactory model for PN G135 can be obtained in which elemental abundances are nearly the same as those of a new detailed model for K 648, the prototypical GHPN in the old globular cluster M 15 (with 12 + log (O/H) = 7.58 ⌠1/13 solar), although C/O may be smaller. Nonetheless, given the paucity of argon and iron in the nebula, PN G135 is likely to be a more extreme Population II object than K 648, reinforcing the idea of an endogenous origin for part of the oxygen in very metal-poor PNe. Assuming a standard H-burning post-Asymptotic Giant Branch evolution, timescale and spectroscopic considerations lead to an optimal solution, in which the distance to PN G135 is 8 kpc, the effective temperature of the nucleus slightly less than 1.3 Ă 105 K, its luminosity 1.4 Ă 1037 erg s-1, its mass 0.59 Mâ, the age of the ionized shell 104 yrs, the ionized mass 0.05 Mâ and the abundances by number (H:He:C:O:Ne) = (106:81 500:90:30:4.5), with C/H being rather an upper limit and O/H and Ne/H uncertain by ±0.3 and ±0.1 dex respectively. Line intensities that could be used as diagnostics of the nebular elemental abundances are provided. Detailed imaging together with ultraviolet and very deep far-red spectra of PN G135 will be essential to definitely narrow the range of acceptable parameters and help us decide whether this exceptional PN is so oxygen-poor as to possibly influence current views on stellar evolution
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
NGC 2579 and the carbon and oxygen abundance gradients beyond the solar circle
We present deep echelle spectrophotometry of the Galactic HII region NGC
2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual
Echelle Spectrograph in the 3550--10400 \AA\ range. This object, which has been
largely neglected, shows however a rather high surface brightness, a high
ionization degree and is located at a galactocentric distance of 12.4 0.7
kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of
the gas phase radial abundance gradients in the outer disc of the Milky Way. We
derive the physical conditions of the nebula using several emission
line-intensity ratios as well as the abundances of several ionic species from
the intensity of collisionally excited lines. We also determine the ionic
abundances of C, O and O -- and therefore the total O
abundance -- from faint pure recombination lines. The results for NGC 2579
permit to extend our previous determinations of the C, O and C/O gas phase
radial gradients of the inner Galactic disc (Esteban etal. 2005) to larger
galactocentric distances. We find that the chemical composition of NGC 2579 is
consistent with flatten gradients at its galactocentric distance. In addition,
we have built a tailored chemical evolution model that reproduces the observed
radial abundance gradients of O, C and N and other observational constraints.
We find that a levelling out of the star formation efficiency about and beyond
the isophotal radius can explain the flattening of chemical gradients observed
in the outer Galactic disc.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
VLT/FORS1 spectrophotometry of the first planetary nebula discovered in the Phoenix dwarf galaxy
Context: A planetary nebula (PN) candidate was discovered during FORS imaging of the Local Group dwarf galaxy Phoenix.
Aims: We use this PN to complement abundances from red-giant stars.
Methods: FORS spectroscopy was used to confirm the PN classification. Empirical methods and photoionization modeling were used to derive elemental abundances from the emission line fluxes and to characterize the central star.
Results: For the elements deemed most reliable for measuring the metallicity of the interstellar medium (ISM) from which the PN formed, [O/H] ⌠â0.46 and [Ar/H] ⌠â1.03. [O/H] has lower measurement errors but greater uncertainties due to the unresolved issue of oxygen enrichment in the PN precursor star.
Conclusions: Earlier than 2 Gyr ago (the lower limit of the derived age for the central star) the ISM had Z = 0.002â0.008, a range slightly more metal-rich than the one provided by stars. Comparing our PN-to-stellar values to surveys of other dwarf Local Group galaxies, Phoenix appears to be an outlier
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
Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18
Photoionization models so far are unable to account for the high electron
temperature Te([O III]) implied by the line ratio [O III]4363A/[O III]5007A in
low-metallicity blue compact dwarf galaxies, casting doubts on the assumption
of photoionization by hot stars as the dominant source of heating of the gas in
these objects. Combinations of runs of the 1-D photoionization code NEBU are
used to explore alternative models for the giant H II region shell I Zw 18 NW.
Acceptable models are obtained, which represent schematically an incomplete
shell comprising radiation-bounded condensations embedded in a low-density
matter-bounded diffuse medium. The thermal pressure contrast between gas
components is about a factor 7. The diffuse phase can be in pressure balance
with the hot superbubble fed by mechanical energy from the inner massive star
cluster. The failure of previous modellings is ascribed to (1) the adoption of
an inadequate small-scale gas density distribution, which proves critical when
the collisional excitation of hydrogen contributes significantly to the cooling
of the gas, and possibly (2) a too restrictive implementation of Wolf-Rayet
stars in synthetic stellar cluster spectral energy distributions. A neutral gas
component heated by soft X-rays, whose power is less than 1% of the star
cluster luminosity and consistent with CHANDRA data, can explain the
low-ionization fine-structure lines detected by SPITZER. [O/Fe] is slightly
smaller in I Zw 18 NW than in Galactic Halo stars of similar metallicity and
[C/O] is correlatively large. Extra heating by, e.g., dissipation of mechanical
energy is not required to explain Te([O III]) in I Zw 18. Important
astrophysical developments are at stakes in the 5% uncertainty attached to [O
III] collision strengths.Comment: 20 pages, 8 figures, to be published in A&
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Detection of [O I] 63 <i>Ό</i>m in absorption toward Sgr B2
A high signal-to-noise 52-90 ÎŒm 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 ÎŒm line is detected in absorption even at the grating spectral resolution of 0.29 ÎŒm. 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
Chemical evolution of the Small Magellanic Cloud based on planetary nebulae
We investigate the chemical evolution of the Small Magellanic Cloud (SMC)
based on abundance data of planetary nebulae (PNe). The main goal is to
investigate the time evolution of the oxygen abundance in this galaxy by
deriving an age-metallicity relation. Such a relation is of fundamental
importance as an observational constraint of chemical evolution models of the
SMC. We have used high quality PNe data in order to derive the properties of
the progenitor stars, so that the stellar ages could be estimated. We collected
a large number of measured spectral fluxes for each nebula, and derived
accurate physical parameters and nebular abundances. New spectral data for a
sample of SMC PNe obtained between 1999 and 2002 are also presented. These data
are used together with data available in the literature to improve the accuracy
of the fluxes for each spectral line. We obtained accurate chemical abundances
for PNe in the Small Magellanic Cloud, which can be useful as tools in the
study of the chemical evolution of this galaxy and of Local Group galaxies. We
present the resulting oxygen versus age diagram and a similar relation
involving the [Fe/H] metallicity based on a correlation with stellar data. We
discuss the implications of the derived age-metallicity relation for the SMC
formation, in particular by suggesting a star formation burst in the last 2-3
Gyr.Comment: 11 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
The Planetary Nebula population of the Sagittarius Dwarf Spheroidal Galaxy
The identification of two new Planetary Nebulae in the Sagittarius Dwarf
Spheroidal Galaxy (Sgr) is presented. This brings the total number to four.
The first, StWr 2-21, belongs to the main body of Sgr. The second, the halo
PN BoBn 1, has a location, distance and velocity in agreement with the leading
tidal tail of Sgr. We estimate that 10 per cent of the Galactic halo consists
of Sgr debris. The specific frequency of PNe indicates a total luminosity of
Sgr, including its tidal tails, of M_V=-14.1. StWr 2-21 shows a high abundance
of [O/H]=-0.23, which confirms the high-metallicity population in Sgr uncovered
by Bonaficio et al. (2004). The steep metallicity--age gradient in Sgr is due
to ISM removal during the Galactic plane passages, ISM reformation due to
stellar mass loss, and possibly accretion of metal-enriched gas from our
Galaxy. The ISM re-formation rate of Sgr, from stellar mass loss, is 5 X 10^-4
M_sun yr^-1, amounting to ~10^6 M_sun per orbital period. HST images reveal
well-developed bipolar morphologies, and provide clear detections of the
central stars. All three stars with deep spectra show WR-lines, suggesting that
the progenitor mass and metallicity determines whether a PN central star
develops a WR spectrum. One Sgr PN belongs to the class of IR-[WC] stars.
Expansion velocities are determined for three nebulae. Comparison with
hydrodynamical models indicates an initial density profile of rho ~ r^-3. This
is evidence for increasing mass-loss rates on the AGB. Peak mass-loss rates are
indicated of ~ 10^-4 M_sun yr^-1. The IR-[WC] PN, He 2-436, provides the sole
direct detection of dust in a dwarf spheroidal galaxy, to date.Comment: 16 pages. MNRAS, accepted for publicatio
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