A well constrained photoioniza
tion model for M43
photoionization
(the nearest spherical HII region)
Sergio Simón-Díaz1
Grazyna Stasinska2
(2)
(1) Instituto de Astrofísica de Canarias
[email protected],,
[email protected]
César Esteban1,3
Jorge García-Rojas1
LUTH. Observatoire de Meudon
[email protected]
grazyna
[email protected],,
[email protected]
jogarcia
@iac.es,,
(3)
[email protected]
cel
@iac.es,,
Artemio Herrero1,3
Departamento de Astrofísica (ULL)
[email protected]
Our goal
M43 is a spherical HII region located at the SW of the Orion Nebula.
Nebula. It is ionized by a single star (HD37061, B1V). We have selected
this well resolved HII region to work in a tailored model of the nebula. We have compared photoionisation code results with man
man y
different observables:
observables: a) spectral parameters from the spectroscopic analysis of the ionising star, b) photometric and morphological
nebular parameters from narrow band imagery and c) results from longslit nebular spectral observations.
Our tools
Teff
FASTWIND
SED
Q(H)
WMBASIC
loggg
log
FASTWIND is optimized for optical analysis, but it has an aproximated treatment of the line blanketing in the FUV.
WMBASIC is optimized for the FUV, however it needs the stellar parameters derived from the optical analysis. A
combined use of these two codes is needed for our purpose.
MODELED
INDICATORS
Abundances He,
He, N, O, S, Ne
Ne,, Fe, Ar ...
FASTWIND (Santolaya-Rey et al. 1997, Repolust et al. 2004), WMBASIC (Pauldrach et al. 2001) and CLOUDY
(Ferland et al. 1998) are the three codes we use for our studies. The formers are spherically extended NLTE line
blanketed stellar atmosphere codes, the latter is a nebular photoionization code.
CLOUDY
R*
L(H) , SH(r)
Figure 1 shows all the ingredients needed and how the squeezers are used in our study.
Density profile
-. Figure 1
RNEB
The observational constraints
What do we know from the study of the star?
What can the images tell us?
us?
Images of the nebula in several narrow filters (H , [OIII]5007, [SII]6725 were taken with the INT+WFC. Three
observables could be obtained from the Himage: the size of the nebula,
nebula the total luminosity emitted in H – LH
– and the surface brightness profile – SH (r) –.
The surface brightness is related with the H density of the nebula through SH (r) 
A density law can be
derived considering a spherical HII region. We have tried different density laws,
laws finding that a r- law gives the
best fitting to the surface brightness profile.
NH2.
A spectrum of HD37061 in the 4000 – 5000 A + H region was obtained with the INT+IDS.
The ionizing flux distribution (SED) of HD37061 can be modelled with WMBASIC.
WMBASIC However, this code needs the
stellar parameters (Teff, logg, (He) and R*) as input. These parameters were derived through the analysis of the
stellar spectrum by means of a visual fitting of the HI, HeI and HeII observed stellar lines with a grid of modelled
FASTWIND lines (see Figure 4). Once the SED was obtained, the number of ionizing photons,
photons Q(H0), could be
calculated.
HD37061
LH total
Teff = 31000 K
logg = 4.1 dex
Size of the nebula
in H filter
Size of the nebula
in H filter
(A)
Av = 1.74  0.10
Mv = -3.1  0.4
Table 1: Information obtained through
the study of the WFC images
Teff = 31500  1000K
Table 3: Stellar
parameters and
number of ionizing
photons derived
from the analysis
of the spectrum
of HD37061.
LH  (1.00  0.23) x 1035 erg s-1
NH = N0 (r/B)-
Through a combined analysis of the nebular spectral
lines ratios [NII] 5755/6584 and [SII] 6716/6731 an
electron temperature and density of Te([NII]) = 7800 K
and Ne ([SII]) = 580 cm-3 have been derived.
R = 5.7  1.0 R
log(L/L) = 4.42  0.17
M = 15  13 M
R(H) = 0.35  0.04 pc
Figure 4: Visual fitting of the HI, HeI and HeII observed stellar
lines (----) with synthetic lines obtained FASTWIND (----, ----)
* A distance of 450  50 pc to the nebula has been considered
In a first step we have used the
nebular abundances she derived
as input in CLOUDY.
CLOUDY
See M. Rodríguez (1999, 2002)
------------- log(X/H) ------------O: -3.56
Fe: -5.91
N: -4.36
Ne: -4.22*
S: -5.16
Cl: -6.87
He: -1.01*
C: -3.58*
* From Orion Nebula (Esteban et al. 2004)
We have also used her measured line
intensities for obtaining the observed
spectroscopic indicators that were
compared with the CLOUDY modelled
ones. Table 4 shows the observed
indicators used in this study. They
have been classified according to
what they probe (Te, Ne, hardness of
stellar radiation, typical ionization
parameter U, abundances. The last
two constraints were obtained from
the study of the H image (see
above).
Te
Ne
Q(He0)/Q(H0)
Q(He0)/Q(H0) & U
Abundances
Other
[NII] 5755 /  6584
[SII]  6717 /  6731, [ClIII] 5538 /  5518
HeI  5876 / H
[SIII]  6312 / [SII]  6717
[OIII] 5007 / [OII] 
7320+30
7320+30
[OII] 
7320+30
7320+30 / [OI]  6300
[ClIII]  5518 / [ClII]  8579
[SII]  6731, [SIII]  6312, [OI]  6300,
[OII] 
7320+30,
7320+30, [OIII] 5007,
[ClII]  8579, [NII]  6584
RH LH, LH (slit
slit))
Table 4: Observed line ratios, as well as other constraints, used as
indicators of the goodness of the CLOUDY models
Comparing observations with CLOUDY (v.95.06 beta5) models of M43
Slit correction
Before to constraint the density ...
At the moment, we have only compared the line intensity
ratios for the slit 1 (see Rodriguez 1999) with the
results from CLOUDY for a central line of sight. The slit
correction has been calculated exactly (the radial
CLOUDY emmisivities have been passed through a slit
located at the same position and with the same size
that the observation one)
Before considering models with the density
law defined in Table 1 we have done a previous
study with constant density models (Figure
5).
5) These models were only calculated for
testing reasons, Density law models are
presented in Figure 6
(H) = 4.56 x 10-25 erg s-1 cm3
 B (H0,T) = 3.17 x 10-13 cm3s-1
log(Q(H0)) = 47.06 +-0.280.15
LH ~ 0.15 – 0.65 B Q(H0)
From the comparation between the total nebular LH
and Q(H0), it has been derived that 1515-65 % of the
ionis
ionising photons escape from the HII region.
Several different CLOUDY models have been studied.
A density bounded model with a density law ~r is
compatible with the observed indicators.
The main discrepancy of the models with the
observations is the [OIII]/H
[OIII]/H ratio,
ratio which is predicted
lower than observed (at least in the central zone,
which is the only one we studied so far)
Whether this is due to the fact we use too soft a
radiation field or a not perfectly adequate density
distribution remains to be investigated. Further
modelling is being carried out.
Figure 6: Density law models
Density law models
This graph compares the best constant density model with
density r -law models.
models. Model () consider the density law
shown in Table 1 and  = 1. Better agreement between model
() and observation is found when a  = 0.09 is considered
(and the same N02
than
than previous model,
model, see Stasinska et al.
(2004)). This last model () gives very similar results than
best constant density model (). The HeI/H
HeI/H problem seems
to be solved
solved,, however the [OIII]/
[OIII]/H
H line ratio remain very low in
this model.
model.
Thank you very much
The main discrepancy comes from the HeI/H
HeI/H and [OIII]/
[OIII]/H
H
ratios
LH= (1.00  0.23) x 1035 ergs-1
We have analised the optical spectrum of the ionizing
star, deriving the stellar parameters, the number of
ionizing photons and the ionizing spectral flux.
Constant density models
First set of models consider a constant density
(Ne = 580 cm-3, derived from the [SII] nebular lines).
All models give very good agreement for the Te and Ne
indicators. Models (, , ) have  = 1, 0.50, 0.10
respectively and are ionisation bounded. These three
models result in a higher LH than the observed one
(indicating that some ionising photons escape from
the nebula). The best way to solve this problem is to
fix the external radii of the nebula and vary the filling
factor to find the best agreement in LH. It
It is found
for models with a filling factor 
=
= 0.10 – 0.12 (, ).
A little sip before
the drink:
drink: some
photons escape
from the nebula
What did
did we get?
We have followed the strategy presented in Stasinska et al. (2004). The aim is to reproduce all the observational constraints (see Table 4) within a certain degree of
tolerance. The parameter p is defined as [log(x
[log(xmod) - log(x
log(xobs)]/t,
)]/t with x being the considered indicator and t depending on the errors for each constraint. We have
assumed tR = 0.05 (~11%) , tLH
LH
 , LH
LH
 ( slit
slit)) = 0.09 (~23%), and t= 0.10 (~25%) for the remaining indicators.
Figure 5: Constant density models
logg = 4.1  0.1 dex
(He) = 0.10 dex
Q(H0) = 10–47.06 ph/s
 =0.5, N0= 58 cm-3, B= 2.36 pc
The longslit nebular spectrum: indicator of chemical composition and other stellar/nebular
stellar/nebular characteristics
We have made use of the nebular spectroscopic data of
M43 from M. Rodriguez thesis for our study (see also
M. Rodríguez 1999). She presented the nebular
spectrum in 5 slit positions inside the nebula. This has
been very useful for our study, as we could test the
photoionization models at different distances to the
star.
Table 2: Photometric data for
HD37061. A d=450  50 pc
has been adopted
logg = 4.1 dex
(C)
(B)
Figure 3: (A) Surface brightness profile in
several filters, (B) Radial profile of the
LH (C) Fit of the H surface brightness
profile with a theoretical one considering a
r- density law and a spherical geometry.
Figure 2: Images of the nebula in several filters
Teff = 32000 K
mv = 6.87  0.05
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Pauldrach, A.W.A., Hoffmann, T.L., Lennon, M., 2001, A&A, 375, 161
Rodríguez, M., 1999, A&A, 351, 1075
Rodríguez, M., 2002, A&A, 389, 556
Santolaya-Rey A. E., Puls, J., Herrero, A., 1997, A&A, 323, 488
Stasinska, G., Gräfener, G., Peña, M., et al., 2004, A&A, 413, 329
Repolust, T., Puls, J., Herrero, A., A&A, 2004, 415, 349