690 research outputs found
No temperature fluctuations in the giant HII region H 1013
While collisionally excited lines in HII regions allow one to easily probe
the chemical composition of the interstellar medium in galaxies, the possible
presence of important temperature fluctuations casts some doubt on the derived
abundances. To provide new insights into this question, we have carried out a
detailed study of a giant HII region, H 1013, located in the galaxy M101, for
which many observational data exist and which has been claimed to harbour
temperature fluctuations at a level of t^2 = 0.03-0.06. We have first
complemented the already available optical observational datasets with a
mid-infrared spectrum obtained with the Spitzer Space Telescope. Combined with
optical data, this spectrum provides unprecedented information on the
temperature structure of this giant HII region. A preliminary analysis based on
empirical temperature diagnostics suggests that temperature fluctuations should
be quite weak. We have then performed a detailed modelling using the pyCloudy
package based on the photoionization code Cloudy. We have been able to produce
photoionization models constrained by the observed Hb surface brightness
distribution and by the known properties of the ionizing stellar population
than can account for most of the line ratios within their uncertainties. Since
the observational constraints are both strong and numerous, this argues against
the presence of significant temperature fluctuations in H 1013. The oxygen
abundance of our best model is 12 + log O/H = 8.57, as opposed to the values of
8.73 and 8.93 advocated by Esteban et al. (2009) and Bresolin (2007),
respectively, based on the significant temperature fluctuations they derived.
However, our model is not able to reproduce the intensities of the oxygen
recombination lines . This cannot be attributed to observational uncertainties
and requires an explanation other than temperature fluctuations.Comment: accepted in Astronomy & Astrophysic
Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (Vexp) of different ions and line widths at the base of
the lines are measured and analyzed for 24 PNe with [WC]-type nuclei (WRPNe), 9
PNe ionized by WELS (WLPNe) and 14 ordinary PNe. A comparative study of the
kinematical behavior of the sample clearly demonstrates that WRPNe have in
average 40-45% larger Vexp, and possibly more turbulence than WLPNe and
ordinary PNe. WLPNe have velocity fields very much alike the ones of ordinary
PNe, rather than the ones of WRPNe. All the samples (WRPNe, WLPNe and ordinary
PNe) show expansion velocities increasing with age indicators, for example
is larger for low-density nebulae and also it is larger for nebulae
around high-temperature stars. This age effect is much stronger for evolved
WRPNe, suggesting that the [WC] winds have been accelerating the nebulae for a
long time, while for non-WRPNe the acceleration seems to stop at some point
when the star reaches a temperature of about 90,000-100,000 K. Non-WR nebulae
reach a maximum Vexp < 30 km/s while evolved WRPNe reach maximum Vexp of about
40 km/s. For all kind of objects (WRPNe and non-WRPNe) it is found that in
average Vexp(N+) is slightly larger than Vexp(O++), indicating that the nebulae
present acceleration of the external shells.Comment: 20 pages, 11 fig. To appear in Revista Mexicana de Astronomia y
Astrofisica, April 200
Photoionization models of the CALIFA HII regions. I. Hybrid models
Photoionization models of HII regions require as input a description of the
ionizing SED and of the gas distribution, in terms of ionization parameter U
and chemical abundances (e.g. O/H and N/O). A strong degeneracy exists between
the hardness of the SED and U, which in turn leads to high uncertainties in the
determination of the other parameters, including abundances. One way to resolve
the degeneracy is to fix one of the parameters using additional information.
For each of the ~ 20000 sources of the CALIFA HII regions catalog, a grid of
photoionization models is computed assuming the ionizing SED being described by
the underlying stellar population obtained from spectral synthesis modeling.
The ionizing SED is then defined as the sum of various stellar bursts of
different ages and metallicities. This solves the degeneracy between the shape
of the ionizing SED and U. The nebular metallicity (associated to O/H) is
defined using the classical strong line method O3N2 (which gives to our models
the status of "hybrids"). The remaining free parameters are the abundance ratio
N/O and the ionization parameter U, which are determined by looking for the
model fitting [NII]/Ha and [OIII]/Hb. The models are also selected to fit
[OII]/Hb. This process leads to a set of ~ 3200 models that reproduce
simultaneously the three observations.
We find that the regions associated to young stellar bursts suffer leaking of
the ionizing photons, the proportion of escaping photons having a median of
80\%. The set of photoionization models satisfactorily reproduces the electron
temperature derived from the [OIII]4363/5007 line ratio. We determine new
relations between the ionization parameter U and the [OII]/[OIII] or
[SII]/[SIII] line ratios. New relations between N/O and O/H and between U and
O/H are also determined.
All the models are publicly available on the 3MdB database.Comment: Accepted for publication in A&
ISO spectroscopy of compact HII regions in the Galaxy. II Ionization and elemental abundances
Based on the ISO spectral catalogue of compact HII regions by Peeters et al.
(2001), we present a first analysis of the hydrogen recombination and atomic
fine-structure lines originated in the ionized gas. The sample consists of 34
HII regions located at galactocentric distances between Rgal = 0 and 15 kpc.
The SWS HI recombination lines between 2 and 8 mum are used to estimate the
extinction law at these wavelengths for 14 HII regions. An extinction in the K
band between 0 and 3 mag. has been derived. The fine-structure lines of
N, O, Ne, S and Ar are detected in most of the sources. Most of these elements
are observed in two different ionization stages probing a range in ionization
potential up to 41 eV. The ISO data, by itself or combined with radio data
taken from the literature, is used to derive the elemental abundances relative
to hydrogen. The present data thus allow us to describe for each source its
elemental abundance, its state of ionization and to constrain the properties of
the ionizing star(s).Comment: Accepted in Astronomy and Astrophysics, 22 pages, 20 figures, 9
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