On the evolutionary state of MWC349

1
On the evolutionary state of MWC349
Christian Hengel, J.W. Goethe Universität
Frankfurt a. M., Germany (hengel_at_em.uni-frankfurt.
de) Clemens Thum, IRAM, Grenoble, France
(thum_at_iram.fr)
Analogies with HD45677 We have used our ISO
spectra to assemble the broadband continuum
spectral energy distribution of MWC349. After
removing all clearly detected spectral lines, the
data were smoothed to an effective spectral
resolution ?/?? 100. Combined with longer
wavelength data taken from the literature, the
SED is shown in Fig.4. The new finding is that
emission from dust is now detected up to about
150-250 µm, beyond which wavelength the emission
is dominated by free-free radiation of the
ionized wind. Although the well defined crossover
wavelength is interesting in itself (it shows
that disk material of lower temperature exists
than known so far), we use this feature here in a
search for MWC349 analogues. The emission line
star HD45677 may well be such an analogue.
Although it is not formally listed as a Herbig
AeBe star, since it is not associated with
nebulosity, violent accretion events have been
observed (Grady et al. 1986,1993), leaving little
doubt that this star is young (Brown et al.
1995). Like MWC349, the star has a strong IR
excess which was observed with ISO SWS. With the
goal to strengthen the analogy even further, we
embarked on a search for an ionized wind (Thum et
al. 2005). This wind was detected with the IRAM
30m telescope at 1.3mm, its size and spectrum
measured with the VLA and the IRAM interfero-
meter (3mm). Together with the submm data
obtained with SCUBA at the JCMT, the picture of
a powerful dense wind emerges. As in MWC349, the
wind is slow, 30 km/s, as shown by the width of
the mm recombination lines detected by us.
Together with other evidence that the star's IR
excess arises in an edge-on circumstellar disk,
HD45677 may be taken as the closest analog of the
enigmatic MWC349.
Summary We present new observational evidence
that MWC349, the bright radio star with a
featureless optical spectrum, is a young massive
star. The new evidence comes from (i) a
quantitative analysis of many fine structure
lines in the star's ISO spectrum, and (iI) a
detailed comparison of the IR to radio spectral
energy distributions of MWC349 and HD45677, a
well known young B2eV star. Twenty fine
structure lines were detected in MWC349 and
analysed together with more than 10 upper limits
with Cloudy. The effective temperature of the
star is 32500 K, and we find the abundances of
all elements with several unambiguously detected
lines (N, O, Ne, S, and Ar) to be solar within
observational error. This analysis rules out
scenarios where a massive post--mainsequence
star expells processed (e.g. enriched in N)
stellar material into a circumstellar shell, and
it suggests that MWC349 is young. The spectral
energy distributions of MWC349 and HD45677 where
we have recently detected an ionized wind are
remarkably similar. As in MWC349, the HD45677
wind is slow. The wavelength where the wind is as
bright as the circumstellar dust is near 200 µm
in both cases. We consider this as further
evidence that MWC349 is also young, still
surrounded by its natal accretion disk and
possibly still accreting.
The ISO spectrum

Fig.1 shows the unsmoothed spectrum extending
from 2.4 to 200 µm. Numerous emission lines are
superimposed on the continuous spectrum of warm
circumsteller dust. Most features are
recombination lines of hydrogen or helium,
followed in number by 20 clearly detected atomic
fine structure lines. Line parameters were
derived using ISAP tools, and the flux
calibration was checked against a few strong
lines detected from the ground (Ne, Br-a), and
was found consistent within 20 percent. At the
nominal ISO spectral resolution, the lines are
not resolved, except for a few strong lines
shortward of 5 µm where the spectral resolution
is highest. Br-a, for example, is slightly
broadened by ca. 100 km/s. No broadening was
found for the fine structure lines. The strongest
limit, 150 km/s, was obtained for Ne at 12.8 µm
the strongest line in the spectrum.
The enigmatic MWC349 The optically
inconspicuous stellar object MWC349 is the
brightest radio star at millimeter wavelengths
and one of the brightest sources in the mid
infrared. Although the complete absence of
photospheric absorption lines makes a traditional
classification impossible, the detection of
numerous emission lines (Hamann Simon 1986)
allowed an coarse estimate of the star's
effective temperature (35000K), and the IR
continuum measurements (Harvey et al. 1979) gave
an estimate of its bolometric luminosity (3 104
Lsun). However, in a detailed analysis of the
optical properties of the star, Cohen et al.
(1985) conclude that "the global properties of
MWC349 cannot be matched by any normal star".
Fig.3 Fine structure lines observed in the ISO
spectrum of MWC349 and modelled with CLOUDY. The
observed line luminosities and their
observational error are shown and connected by a
thick line. Upper limits are given for the lines
not detect- ed. The model results are schown as
filled triangles connected by a dotted line.
Excitation of the circumstellar material A
first qualitative, but robust analysis of the
degree of excitation of the circumstellar shell
can be made with the fine structure lines
observed in the ISO spectrum. Fig. 2 shows all
elements with cosmic abundan- ce larger than 10-7
of hydrogen. The ionization stages whose ground
ground state have fine structure transitions in
the ISO spectrum are shown as square symbols. The
square is filled if the transition is detected,
it is crossed out if not, and it is left open if
data are inconclusive. Fig.2 clearly shows that
the maximum excitation energy is in the range
between 42 and 53 eV. The most significant upper
limit is set by the absence of recombination
lines from He. Stellar photons of higher energy
are rare, indicating an effective temperature
lt40000 K of the stellar radiation field.
Quantitative analysis of fine structure lines
The 20 fine structure lines detected in the ISO
spectrum were combined with upper limits obtained
for 10 more fine struncture transitions, all
listed in the upper part of Fig.2. The observed
line luminosities are shown with their errors
bars, conservatively taken at 50 percent, and
upper limits are drawn for the lines not
detected. For better visibility, these
observations are connect by a thick line.

The 30 transitions were
then analysed with CLOUDY (Ferland 2003). We
adopted a distance of 1.2 kpc for the star and an
extinction of Av10 mag. The distribution of the
circumstellar material was assumed to be
axisymmetric, with the neutral disk in the
equatorial plane subtending an unspecified solid
angle. The ionized material fills the remaining
volume and has a r-2 density dependence where r
is the distance from the central star. The
ionized gas has an inner radius Ri where its
electron density is ne. These latter two
parameters were varied together with Teff, the
effective temperature of the star, and the
elemental abundances. A Kurusz-type stellar
atmosphere was assumed throughout.

In a first attempt, we fitted the
line ratios of those ions which have transitions
at different ionization stages, e.g.
SIII?18.7/SIV ?10.5?m. Although these ratios
could always be reproduced, we soon found that a
simultaneous fit of the luminosities of lines
from the wide range of ionization stages was only
possible with a 2-component model. The high (HD)
and low (LD) density components were each
described by their inner radius Ri, their density
ne at Ri, and volume filling factor f. We
consider the HD component to be representative of
a line of sight from the star in a direction
close to the neutral disk, i.e. skimmimg through
the dense disk corona. The LD component then
samples the bulk of the ionized wind. The outer
radius is at infinity for both components.
Satisfactory fits were then obtained for
virtually all 30 transitions. Table 1 gives the
parameters of our best fit model. Table 1 Best
fit CLOUDY model component ne cm-3 Ri cm
f Av mag Teff K el.
abundances LD 106 1015
0.99 HD 4107 21015 0.01
We emphasize that the assumption of an
axisymmetric shell with only two ionized
components and a single inner radius is probably
an oversimplification. In particular, we believe
that the sphere inside Ri is not really empty.
The disk inside Ri may merely be very thin, so
that stellar photons reach the HD component, and
a forteriori the LD, component with negligible
extinction. The main conclusion however is that
the best fit model is compatible with solar
abundances. Elements with several unambiguously
detected lines (N, O, Ne, S, and Ar) have solar
abundances within observational error. Data for a
few other elements (K, Na, Cl, and P) are fully
compatible with solar abundances, with the only
and notable exceptions of Si and Fe which are
underpredicted by our 2-component model. This
analysis rules out scenarios where a massive
post-main-sequence star expells processed (e.g.
enriched in N) stellar material into a
circumstellar shell, and it suggests that MWC349
is young.
Fig.4 The spectral energy distributions of
MWC349 and HD45677, a young star of internediate
mass. Data points at long wavelengths (Thum et
al. 2005) were obtained with IRAM telescopes
(squares), JCMT (dots), and the VLA (triangles).
We compare the continuous spectral energy
distributions of the two stars in Fig.4. We note
that our detection of an ionized wind allows to
place the crossover wavelength (dust as bright as
the wind) again near 200 microns. Even some
details, like some broad band features of the
dust spectrum, also appear to repeat. The close
analogy between the two stars, taken together
with the established youth of HD45677, then
clearly suggests that MWC349 is also young and
possibly still accreting.



Conclusions Our analysis of the 20 fine
structure lines detected in the ISO spectrum of
MWC349 shows that the star has an effective
temperature of 32500 K and a circumstellar
ionized shell which is well modelled by two
components with r-2 density laws and electron
densities of 106 and 4107 cm-3 at their inner
radius. The abundances of the most abundant
elements, most notably nitrogen, are solar. We
take this as evidence that the circumstellar disk
from where the wind is likely to arise is not
formed by mass loss of processed, e.g.
N-enriched, stellar material. The simpler
interpretation is that the disk is formed by
accretion and that MWC349 is young. The youth of
the star is further supported by the similarity
of its SED to that of HD45677. Both stars have
circumstellar dust disks of similar temperature.
They also both have strong and slow ionized
winds. Our new wind observations show that the
wavelength where dust and wind emission are
comparable is near 200 µm in both stars. Given
the established youth of HD45677 where violent
accretion events have been directly observed,
MWC349 may also be young and possibly still
accreting. Acknowledgement we are greatful to
Dr. Kegel (Universtät Frankfurt a.M.) and Dr.
Grewing (IRAM) for their interest in and support
of this work, and to Dr. Ferland for advice with
CLOUDY. C.H. thanks Dr. Mauersberger,
IRAM-Granada, for his hospitality and Dr. Sievers
for help with computational aspects of this
work. References Brown T.M., Buss R.H.Jr, Grady
C., Bjorkman K. 1995, ApJ 440, 865-873 Cohen, M.,
Bieging, J.H., Dreher, J.W., Welch, W.J. 1985,
ApJ 292, 249 Ferland, G.J. 2003, ARAA 41,
517 Grady, C.A., Bjorkman, K.S., Sheperd, D. et
al. 1993, ApJ 415, L39 Grady C.A., Bjorkman K.S.,
Shepherd D., Schulte-Ladbeck R.E., Perez
M.R., de Winter D., Thé P.S. 1993, ApJ 415,
L39 Hamman, F. Simon, M. 1986, ApJ 909,
311 Harvey, P.M., Thronsom, H., Gatley, I. 1979,
ApJ 231, 115 Hengel, C., Kegel, W.H. 2000, AA
361, 1169 Lamers H.J.G.L.M., Zickgraf F.J., de
Winter D., Houziaux L., Zorec J 1998,
AA, 340, 117 Martin-Pintado, J., Bachiller R.,
Thum C., and Walmsley C.M. 1989, AA 215,
L13 Molster F.J., Waters L.B.F.M., Tielens
A.G.G.M., Barlow M.J. 2002, AA 382,
184 Thum C., Martin-Pintado J., Quirrenbach A.,
and Matthews H.E. 1998, AA 333,
L63-66
Thum,C., Matthews, H.E.,
Morris, D., Claussen, M.C. 2005, in preparation


Fig.1 ISO spectrum of MWC349. Numerous emission
lines are superimposed on the continuous spectrum
of warm circumsteller dust. Most features are
recombination line of hydrogen and helium,
followed in number by 20 atomic fine structure
lines. The bightest line of all is the 2P3/2 ?
2P1/2 transition at 12.8 µm.
Consequently, the evolutionary state of MWC349
has been debated ever since. In a more recent
attempt on an improved classification Lamers et
al. (1988) asssign to MWC349 a spectral type of
O9IIIe and summarize their view on the nature
of the stars as "sgBe? /HAeBe?". In general,
arguments put forward in favor of a
pre-main-sequence (pre-MS) state include (i) the
presence of a bright and large circumstellar
disk, and (ii) the location very near the
galactic plane, in the complex region Cygnus X
where many star formation sites are known to
exist. Arguments in favor of a late (post-MS)
state include (i) the unusually slow speed (50
km/s) of the ionized wind which may be easier to
understand if arising from the extended
atmosphere of a post MS star, and (ii) the
absence of any spectroscopic evidence for
infall. Adding to the enigma of this star is the
detection of radio recombina-tion line masers
(Martin-Pintado et al. 1989) originating in the
circum-stellar disk, so far the only source of
masing recombination lines found. The maser/laser
phenomenon extends from short mm wave- lengths
through the submm, down to the mid IR (Thum et
al. 1998), and was investigted theoretically by
Hengel Kegel (2000). These latter data were
obtained with ISO SWS and LWS full wavelength
scans which we re-analyse here for atomic fine
structure lines.

Fig.2 Fine structure lines of all elements with
cosmic abundance gt10-7 of hydro-gen. Symbols are
drawn at the excitation energies of the lowest
ionization stages. Squares indicate the
ioniza-tion stages with lines in the ISO
spectrum either detec-tions (filled squares), or
non-detection (square with cross), or an
inconclusive result (empty square) due to
instabilities in the data. Triangles indicate
ionization stages either without fine structure
splitting (open triangles) or with transitions
outside the ISO range (filled triangles). The
dashed verti-cal line is drawn at 13.6 eV, and
the dash-dotted lines limit the range where the
photon energy in MWC 349 cuts off.

10 32500 solar 50