The infrared excess (IRE) of the Large Magellanic Cloud (LMC) HII nebulae is found to correlate positively with the temperature of the ambient radiation field or with the He(+)/H(+) abundance ratio. This result is discussed in terms of a selective absorption of the photons in the range 504 to 912 A relative to the He ionizing photons. This interpretation may explain the paradox of finding highly excited nebulae with only relatively moderate equivalent width of their Balmer lines

Ungerer, V.

Viallefond, F.

English

NASA Technical Reports Server

N87- 24298
INFRARED EMISSION AND EXCITATION IN LMC HII REGIONS
V. Ungerer
Ruimteonderzoek 6roningen, Postbus 800
9700 AV Groningen, The Netherlands
F. Viallefond
DEMIRM, Observatory of Meudon
F-92190 Meudon, France
ABSTRACT. The infrared excess (IRE) ofLMC BII nebulae is found to correlate
positively with the temperature of the ambient radiation field or with the
He+/H + abundance ratio. This result is discussed in terms of a selective
absorption of the photons in the range 504-912_ relative to the He ionizing
photons. This interpretation may explain the paradox of finding highly excited
nebulae with only relatively moderate equivalent width of their Balmer lines.
1. INTRODUCTION
The excitation in HII nebulae is sensitive to the metallicity and to the
energy distribution of the radiation field in the far UV (_ < 912_). The
metallicity as traced by the abundance of oxygen is known to be fairly uniform
across irregular galaxies in contrast to the case of spiral galaxies. Thus
restricting to a sample of HII nebulae in the Large Magellanic Cloud allows the
study of the relative variations of the temperature of the radiation field from
nebula to nebula, minimizing any possible influence of abundance variations.
2. OBSERVATIONAL DATA
The IRAS AO maps of the LMC (P. Schwering, in preparation) have been
analyzed in order to separate the emission originating from sources and the
underlying diffuse large scale component. About 200 sources have been identified
with HII regions outside the 30 Doradus region (Ungerer et al., in preparation).
In this catalogue we have selected the HII regions detected in the 5 6Hz radio
continuum survey (McOee et al. 1972) and having optical spectroscopic
measurements (Page1 et al. 1978 and references herein). We rejected the source
N44 whose radio continuum flux is contaminated by the SNR emission. For the HII
nebula N91 an extended north-south plateau shows up at 5 GHz in contrast with
the infrared and 408 MHz emission. We thus used the 408 MHz flux density. We
have redetermined the electron temperature, the abundance of He + relative to H+
and O/H in a homogeneous way using the atomic data compiled by Mendoza (1983).
The ambient radiation field in an HII nebula can be conveniently characterized
by an effective temperature Tel f which measures the relative number of the
photons shortward of 504A to those from 504 to 912_. Stasinska (private
communication) has demonstrated that this quantity is relevant for the
excitation of nebulae, no matter if there is one or several exciting stars and
which stellar atmosphere model is used. Thus we have defined Tel f for the
ambient field inside a nebula as the Tel f of a stellar atmosphere model having
the same ratio of He to H ionizing photons. Based on the NLTE stellar atmosphere
models from Mihalas (1972) and the grid of photoionization models (Stasinska,
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 247
https://ntrs.nasa.gov/search.jsp?R=19870014865 2020-03-20T10:57:06+00:00Z
V.UNGERERANDF.VIALLEFOND
1982), we assign to each nebula of our.sa.mp.le a Teff' this temperature
reflecting directly the temperature of the xonlzxng star cluster if there was no
dust mixed with the ionized gas. This TA_f has been determined from the relative
intensities of [01113727, [011114363, [O_1YI]4959+5607 and H[_4866, and we checked
that it was also consistent with the He+/H + ratios (Fig. I).
The total infrared emission has been determined by multiplying the sum of
the four in-band fluxes with a bolometric correction factor of 1.8: this factor
is actually insensitive to the dust temperature over a large range of values
from 30 to 200K (Boulanger, private communication) allowing us to get reliable
bolometrlc infrared emission for HII regions. The IRE is deduced: by definition
it has a value of 1 corresponding to all the bolometric infrared luminosity
originating from Lyman a absorption (we further assumed that all Lyman continuum
photons which ionize the gas degrade into Lyman a photons).
3. RESULTS AND DISCUSSION
In Fig. 2 the comparison between the IRE and Tef f is presented. The IRE
tends to increase with the radiation field temperature. More striking is the
very similar correlations in Fig. I and Fig. 2 which indeed implies that the IRE
correlates positively with the He+Ill + ratio.
If the absorption of Lyc photons by dust were negligible, the IRE would
measure directly the contribution of the nonionizing stars relative to the
exciting stars (>15Mo), and we would predict a decrease of the IRE with
increasing Tef f caused either by different Initial Mass Functions (IMF) or by
age effects for the different nebulae. Thus we are forced to conclude that dust
in HII nebulae plays a major role for the excitation, and the IRE may not
directly reflect the properties of the young star cluster. One explanation for
the increase of the IRE with Tef f is the existence of selective absorption of
the 504-912_ photons relative to the He-ionizing photons. If the 504-912_
photons are preferentially absorbed, the effective temperature of the ambient
radiation field will be increased relative to the effective temperature Tif f of
the star cluster. This effect is illustrated in Fig. 3 where we have computed
the IRE based on the NLTE Mihalas atmosphere models and ignoring absorption of
all phqtons l ongward of FI2_. The solid lines are related to the fraction of
504-912_ photons absorbed by the dust, and the dashed lines show the evolution
of Tef f as the amount of selective absorption of Lyc photons increases.
In Tab. 1 we show the effects on derived star cluster properties for two
HII regions with extreme physical conditions when this selective absorption is
considered. The IMF parameters (i.e., the slope x and the upper mass cut-off
mu), duration of star formation _, star formation rate SFR, and the total
luminosity are based on an evolutionary model for a star cluster which gives
T_ff(x,mu,_) and W*_(x,mu,_).
248
LMCHIIREGIONS
TABLE1
Teff(104K)
N214C N105
3.58 3.52 4.3 3.6
IRE (from phot.>912_) 1.7 0.0
Equivalent width W_(_) 115 137
H phot. absorbed by dust 0% 16_
Ionization rate (1050s -1) 1.0 1.0
Stellar Lye rate (1050s-1) 1.0 1.2
IMF upper cut-off mu(Mo) 40 25
CSF duration (Myr) 4.5 0
SFR (103MoMyr -1) 3.9
Stellar mass formed (103M o) 18 3.0
Present day LTot(106Lo) 8.1 8.0
LFIR/LTo t 0.09 0.10
3.7 1.0
85 170
0% 5O%
2.5 2.5
2.5 5.0
140 35
>>10 0
-- 70
-- 25
-- 0.1
Table 1: Correction effects on star cluster properties for the far UV
selective absorption: two columns are given for each nebula corresponding
respectively to the extreme cases of no selective absorption and Zero Age Main
Sequence star clusters. The last six lines of this table are based on a star
cluster model very briefly presented in the paper of Viallefond and Thuan (19831
and revised by using the NLTE Mihalas stellar atmosphere model (Viallefond,
unpublished). An IMF slope of 1.3 was assumed corresponding to the Salpeter IMF
slope for the solar neighborhood: continuous star formation was also assumed. If
these two assumptions are relaxed, the star cluster parameters will be
different: however it will not affect the conclusions of this paper. An
unreasonable solution is indicated by "- -".
Let Ap(radio)-AR(Balmer) be the apparent "excess" of extinction derived
by comparing the extfnotion A_. (radio), obtained from the relative strength of
the H_ Balmer line to the radio free-free emission, with Ap (Balmer), obtained
from the relative strengths of the Ha and Hp Balmer lines (Caplan and Deharveng,
1985). This "excess" extinction is believed to be at least partly caused by
the presence of dust mixed with the ionized gas: it might then be related to the
amount of selective extinction. The "excess" extinction for N214 is only 0.16
mag. compared to 0.54 mag. for N105, consistent with our analysis, which
suggests very little selective absorption for N214 compared to N105. This trend
is supported by the analysis of the full sample. The case of N105 illustrates
how partial correction for selective absorption can solve the problem of finding
many extragalactic HII regions with high effective temperatures (40,000 to
50,000K) and very low values of WB (Stasinska and Viallefond, in preparation).
On the contrary, less excited HI_ regions such as N214C can be interpreted
without any selective absorption. Another important implication which emerges
from this discussion is that a substantial fraction of the nonionizing radiation
may escape the ionized regions contributing to the large scale interstellar
radiation field in the LMC: only 10 to 20% of the total luminosity from the
young star clusters would be degraded into far infrared emission locally inside
the plasma, i.e., in the sites of star formation. Finally it is interesting to
notice that the absorption efficiency of small graphite and silicate grains
(with typical sizes of 30 to 100_) does peak near 700 to 800_ (Draine and Lee,
1984). In this respect, the study of the excitation in HII regions may give
249
V.UNGERERAND F. VIALLEFOND
Fig. 1: Abundance of He + relative to H+ .t2
as a function of the far UV color
termperature of the radiation field in .t,
LMC HII regions. This temperature is
expressed in terms of an effective
temperature as defined in the text. The ,
curve is based on photoionization models
(Stasinska, 1982). -=
.09
.08
.07
Fig. 2: Observed infrared excess as a
function of the temperature of the
radiation field in LMC HII resions: the _ s
sample of nebulae is common with Fig. 1.
All Lyman continuum photons which are
absorbed by the gas are supposed to be _ 4
degraded into Lyman a photons. By
definition the IRE is unity if all the
bolometric infrared emission is entirely _ _
caused by dust absorption of these Lyman
a photons.
Z
Fig. 3: Grid of a simple model for the
IRE i£ it is caused exclusively by dust
absorption of the 504-F12_ photons. The
dashed lines correspond to the evolution
of the IRE and of the radiation field
temperature relative to the amount of
selective absorption. The continuous
lines correspond to different values of
the ratio NL/N, L of the 504-912_
photons, NL being the number of these
photons emitted by the star cluster and
N, L the number of these photons which
effectively ionized the 8as. For an IRE
of unity the radiation field temperature
is equivalent to the star cluster
temperature T_ ff.
.06
3.4
1 I
3.4
' ' ' I ' I ' ' ' I
I,IIC XU Rellion$
i
/
3.6 3.8 4 4.2
Far UV Color Temperature T,n ( 104 K )
' ' ' I ' ' ' I ' ' ' I I ' ' '
L_C HII Relion$
I
4.4
I ,ii , , , I , = , I , , , I , , ,
36 3.8 4 42
Far UV Color Temperature T_ ( I04 K )
6
i ' /' ' '
F l /
I / _s
I T/,_., T
_./ 1 _,l
I /,_ _./ / 1/
" -I /_ ./
:: . / " /
c 1,, /
/' J./,, /
I /' / I ,/ , , 11% , , L.--.,--"I--, I , , _'
3.4 36 38 4 4.2
Far UV Color Temperature T,n ( I04 K )
I
t
I
250
LMCHIIREGIONS
valuable new constraints on the lower part of the size distribution of the grain
population. The 2200_ bump of the extinction curve of the LMC is apparently weak
(Prevot et al. 1984) compared to the bump for the standard galactic curve
(Savage and Mathis, 1979): the properties of the grain population must be
different in the LMC and in the solar neighborhood: selective absorption in the
far UV would be primarily caused by the small silicate grains in the LMC if the
small graphite grains are responsible of the 2200_ bump of the standard
extinction curve.
4. CONCLUS IONS
From a sample of LMC HII regions for which all relevant information is
available, a positive correlation is found between the IRE and the effective
temperature of the ambient radiation field or the He+/H + abundance ratio. We
suggest the presence of selective absorption in the far UV to explain this
observed phenomenon. ]_hile this result introduces a lot of complications for the
interpretation of the IRAS measurements of the young star-forming regions in
galaxies, it may solve the apparent paradox of finding many highly excited HII
nebulae with only very moderate values for their equivalent width of H_.
REFERENCES :
Caplan, J., Deharveng, L. 1985, Ast. Ap. Suppl. 62, 63
Dottori, H.A., Bica, E.L.D. 1981, Astr.Ap.,102, 245
Draine, B.T., Lee, H.M. 1984, Ap.J.,285,89
McGee, R.X., Brooks, J.W., Batchelor, R.A. 1972, Australian _.
Phys.25, 581
Mendoza, C. 1983, in Planetary Nebulae IAU Syrup. 103, p143
Mihalas, D. 1972, Non LTE model atmospheres fo_ B and O stars,
NCAR-_N/str-76
Page1, B.E.$., Edmunds, M.G., Fosbury, R.A.E., Webster, B.L. 1978,
M.N.R.A.S., 184, 569
Prevot, M.L., Lequeux, J., Maurice, E., Prevot, L.,
Rocca-Volmerange, B. 1984, Astr.Ap., 134, 389
Savage, B.D., Mathis, J.S. 1979, Ann.Rev.Astr.Ap., 17, 73
Stasinska, G., 1982, Ast.Ap.Suppl., 48, 299
Viallefond, F., Thuan, T.X. 1983, Ap.J. 269, 444
251


Infrared emission and excitation in LMC HII regions

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