A&A 430, 541-548 (2005)
DOI: 10.1051/0004-6361:20041788
F. Comerón1 - A. Pasquali2
1 - European Southern Observatory, Karl-Schwarzschild-Strasse 2,
85748 Garching, Germany
2 -
Institute of Astronomy, ETH Hoenggerberg, HPF, 8093 Zurich,
Switzerland
Received 4 August 2004 / Accepted 9 September 2004
Abstract
We present the results of a search for the ionizing star
of the North America (NGC 7000) and the Pelican (IC 5070) nebulae
complex. The application of adequate selection criteria to the
2MASS
broad-band photometry allows us to narrow the search
down to 19 preliminary candidates in a circle of
radius containing most of the L935 dark cloud
that separates both nebulae. Follow-up
near-infrared spectroscopy shows that most of these objects are
carbon stars and mid-to-late-type giants, including some AGB
stars. Two of the three remaining objects turn out to be later
than spectral type B and thus cannot account for the ionization of
the nebula, but a third object, 2MASS J205551.25+435224.6, has
infrared properties consistent with it being a mid O-type star at
the distance of the nebulae complex and reddened by
.
We confirm its O5V spectral type by means of visible
spectroscopy in the blue. This star has the spectral type required
by the ionization conditions of the nebulae and photometric
properties consistent with the most recent estimates of their
distance. Moreover, it lies close to the geometric center of the
complex that other studies have proposed as the most likely
location for the ionizing star, and is also very close to the
position inferred from the morphology of cloud rims detected in
radio continuum. Given the fulfillment of all the conditions and
the existence of only one star in the whole search area that
satisfies them, we thus propose 2MASS J205551.25+435224.6 as the
ionizing star of the North America/Pelican complex.
Key words: ISM: HII regions - ISM: individual objects: NGC 7000 - ISM: individual objects: IC 5070 - ISM: individual objects: W80
The North America (NGC 7000) and Pelican (IC 5070)
nebulae are two of the most nearby and well known diffuse,
extended HII regions. Together with the dark lane that separates
them, L935 (Lynds 1962) they form the
W80 complex (Westerhout 1958). The overall
extent and low density of the nebulae indicate that they are
relatively evolved, and the absence of compact or ultracompact
components in them shows that they do not host massive star
formation at present. However, recent and ongoing star formation
at intermediate and low masses is revealed by the presence of very
young Be and T Tauri stars, already noted by Herbig
(1958), and of Herbig-Haro objects (Bally & Reipurth
2003). Although the distance to the complex has been
traditionally controversial (values between 200 and 2000 pc can be
found in the literature; see Wendker et al. 1983
and, more recently, Bally & Reipurth 2003, for
references), observations of large samples of stars in the
direction of the complex (Laugalys & Straizys 2002,
and references therein) conclusively favor a distance of
pc. Their results also demonstrate that both the HII region
and L935 are parts of a single entity.
Despite the notoriousness and the proximity of the North America and
Pelican nebulae, the star or stars responsible for their
ionization have remained unknown until now. The first tentative
identification can be traced back to Hubble (1922), who
proposed Deneb as the excitation source. Realizing the
unsuitability of Deneb's A2Ia spectral type as an ionization
source, Sharpless & Osterbrock (1952) proposed
instead the O6V((f)) close binary HD 199579 as a good
candidate, but its role as the dominant source of ionization was
soon called into question. Herbig (1958) considered it
likely that the true ionizing star is actually hidden by L935. A
ionizing star different from HD 199579 is also suggested by the
geometry (Wendker 1968) and the ionization structure
of the nebula (Goudis & Meaburn 1973;
Goudis 1976), which also suggests a spectral type of
the ionizing star between O4 and O5. Following a different
approach, high resolution radio continuum observations across the
North America/Pelican/L935 complex by Matthews &
Goss (1980) revealed a number of sharp rims probably
caused by the interaction of the radiation from a central source
with dense clumps of gas, similar to the visible bright rims that
stand out in H
images. The positions and morphologies of
the rims led Matthews & Goss (1980) to pinpoint with
great accuracy the likely location of the ionizing star, near the
geometric center of the nebulae. Near-infrared observations of
sources in L935 by Neckel et al. (1980) turned up a
heavily reddened, promising candidate ionizing star that was
however discarded after further observations by Eiroa et al. (1983), who proved it to be an evolved star
unrelated to the complex. Also Bally & Scoville (1980)
produced a list of 11 bright infrared sources among which the
ionizing star may be found.
In this paper we use 2MASS observations to photometrically identify possible early-type stars over a large area of L935 that includes the geometric center of the complex and its surroundings, where the ionizing star is most likely located. Since broad-band near infrared photometry alone cannot unambiguously discriminate reddened early-type stars from some classes of cool, evolved luminous stars, we have also obtained H and K band spectroscopy that rules out a physical relationship of most observed stars with the nebula complex. Spectroscopy narrows down our list of candidates to just one moderately reddened star whose photometric and spectroscopic characteristics are in agreement with those of a mid O-type star at the reported distance of the complex. Indeed, we present spectroscopy in the visible that confirms an O5V spectral type for this star. We thus propose that this star is the true source of the ionization of the North America and Pelican nebulae.
We have carried out our search for the ionizing star of the North
America/Pelican complex in a circular area of the sky
in radius centered on the coordinates
,
,
near the
geometric center of the complex. The chosen radius allows for a
very generous displacement of the ionizing star with respect to
this position. Figure 1 shows that the search area
encompasses most of the L935 cloud.
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Figure 2:
2MASS color-magnitude diagram of all the stars brighter than
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The basic material for our identification of candidate ionizing stars is
the 2MASS all-sky survey. The depth of the survey in the magnitude is by far sufficient for the detection of all the
candidate ionizing stars even if obscured by AV = 35 mag (AK
= 3.9 mag), which is the highest value of the extinction derived
from the maps of Cambrésy et al. (2002).
Figure 2 shows the
,
diagram of all the
2MASS stars brighter than
in the search area. As a
first selection, we considered only those stars whose color and
magnitude are consistent with them being early-type main sequence
stars at the distance of the nebula, assuming an upper limit of
1 kpc to the distance of the complex and a spectral type B0V for
the latest possible ionizing star to allow ample margin for
uncertainty. Taking intrinsic colors and absolute magnitudes from
Tokunaga (2000) and Drilling &
Landolt (2000), and adopting the extinction law of
Rieke & Lebofsky (1985), which is in good agreement
with the 2MASS color-color diagram of the region (Cambrésy et al. 2002), we selected as candidates stars fulfilling
the condition
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(1) |
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(2) |
Spectra of the 19 selected objects were obtained on the night of 30/31 July
2004 from the Calar Alto observatory using MAGIC, the
near-infrared camera and spectrograph at the 2.2 m telescope. The
spectra were obtained using the resin-replica grism providing a
resolution
over the
1.50-2.40
m range with the 1'' slit used. Each star was
observed at six positions along the slit, with exposure times per
position ranging from 20 s (stacking 10 integrations of 2 s) for
the brightest stars to 60 s (stacking 20 integrations of 3 s) for
the faintest. The extraction and calibration of the spectra was
carried out using dedicated IRAF scripts. The frames obtained at
consecutive slit positions were subtracted from each other to
cancel out the sky contribution to the spectrum, and were divided
by a flat field frame. A one-dimensional object spectrum was then
extracted from each sky-subtracted, flat-fielded frame. Wavelength
calibration of each individual spectrum was performed using the OH
airglow lines in each frame as a reference (Oliva &
Origlia 1992). The wavelength-calibrated spectra
extracted at each sky position were then coadded, with deviant
pixels due to detector defects or cosmic ray hits automatically
clipped off. Cancellation of telluric features was achieved by
ratioing the object spectra by those of the nearby G5IV star
HD 190771, which is expected to be featureless at the resolution
employed, reduced in the same manner. Finally, relative flux
calibration was performed by multiplying the reduced spectra by
that of a 5700 K blackbody, which should be a good approximation
to the spectral energy distribution of an unreddened G5IV star in
the wavelength range covered by our spectra.
Spectroscopy in the blue of two selected stars, referred to as #4
and #11 in Sect. 4, was performed on the night of
2/3 August 2004 using CAFOS, the visible imager and spectrograph
at the 2.2 m telescope on Calar Alto. A grism was used covering the
range shortwards of
Å at a resolution of
and with a 1''5 slit. The
observations of Star #4 amounted to 145 min of exposure time
split into five individual exposures. Spectra produced by three
lamps of HgCd, He, and Rb were taken between consecutive exposures
for wavelength calibration, in order to minimize the effects of
instrument flexure. The spectrum of Star #11, much brighter in
the blue, consisted of one single integration of 30 min. The
frames were subtracted from bias and divided by a spectroscopic
flat field, and the spectra were subsequently extracted from each
one of them. The individual wavelength-calibrated spectra of
Star #4 were coadded after identification and removal of cosmic
ray hits. Due to the steep slope of the spectra of these
considerably reddened stars, their continuum was normalized to
unity in order to facilitate feature recognition and comparison to
spectral atlases.
Table 1: Positions, photometry, and near-infrared spectral types of the observed stars.
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Figure 3:
Infrared spectra of carbon stars included in our sample,
characterized by the sharp flux drop at
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Figure 4:
Infrared spectra of mid- or late-type objects included in our
sample other than carbon stars, characterized by the appearance of CO bandheads
longwards of 2.29 ![]() ![]() ![]() |
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Our near-infrared spectroscopic results are summarized in
Table 1. The vast majority of the spectra of the 19
stars observed are found to display the distinctive
characteristics of cool photospheres: CO(2, 0) absorption bands
starting at 2.293 m are visible in the spectra of 16 of the
objects. The abundant presence in our sample of carbon stars (5
objects easily distinguishable by the prominent C2 feature at
1.77
m) and AGB stars (6 objects, also easy to identify due
to the extended wings of the broad water vapor feature separating
the H and K bands) is due to their intrinsic colors similar to
those of early-type stars as noted in Sect. 2. The
misclassification of the first three objects as possible
early-type stars in the photometry-selected sample is due to
saturation effects in the 2MASS K-band photometry; we decided to
keep these three objects in the spectroscopic sample since their
brightness and, in the case of object #2, the reddening, were in
principle compatible with the photometric properties expected for
the ionizing star. The misclassification of object #7 cannot be
explained in those grounds, and it may be due instead to intrisic
infrared excess emission. The spectra of the cool objects, split
between carbon stars on one side, and AGB stars and other
mid-to-late type stars on the other, are shown in
Figs. 3 and 4 respectively.
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Figure 5:
Two stars with early-type spectra ruled out as possible ionizing
sources. Star #9 is an unobscured star classified in the visible as F5V, while
the prominent Br![]() ![]() ![]() |
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Figure 6:
Spectrum of Star #11, showing the Balmer lines H![]() ![]() |
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In principle, the most interesting stars from the viewpoint of our search are
#4, #9, and #11, which display infrared spectra characteristic
of early-type stars. A spectral classification of Star #9 (=HD 199373)
as F5V exists in the literature
(Fehrenbach 1961), which agrees with the spectral
type derived from its Strømgren photometric indices (Jordi et al. 1996). Its color and spectral type suggest that it
is a nearby unreddened star, in agreement with the distance of
70 pc measured by Hipparcos, as well as with the featureless
infrared spectrum that we obtain (Fig. 5). This is
therefore a foreground star whose relationship to the complex
under discussion can be safely ruled out. On the other hand,
Star #11, whose spectrum is also shown in Fig. 5,
displays prominent Br
absorption lines whose equivalent
depths are near the peak defined by early A types (Meyer et al. 1998), indicating a spectral classification between
A0 and A4, which discards it as a possible ionizing star. Its
infrared colors are redder than those intrinsic of a A-type star,
indicating an extinction of
mag. The visible
spectrum, presented in Fig. 6, confirms the early A
type of the star but places it above the main sequence as derived
from its relatively narrow Balmer series lines, lacking the broad
wings that characterize A stars of luminosity class V (e.g.
Jaschek & Jaschek 1987). A main-sequence
classification is also ruled out by the fact that the absolute
magnitude of a A0V star,
MK = 0.65 (Drilling &
Landolt 2000), would imply a distance of only 175 pc.
Since the only candidate to produce such a high obscuration in the
direction of the star is the L935 cloud, a luminosity class V
would require an even closer distance of the cloud from the Sun,
which does not seem plausible in view of other evidence, in
particular the reddening distance measurements of Laugalys &
Straizys (2002) that set a firm lower limit of
400 pc. On the other hand, a comparison to a spectrum of the
nearby A supergiant Deneb obtained with the same instrumental
setup shows that the Balmer lines of Star #11 are broader,
suggesting a luminosity class III for the latter and consistent
with its location behind L935 and the W80 complex.
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Figure 7:
Infrared spectrum of Star #4 = J205551.3+435225, which we propose
as the ionizing star for the North America and Pelican nebulae. The lack of
absorption lines at the resolution of this spectrum is consistent with a O or
early B type, or with a F or later type. The former possibility is favored by
the
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Figure 8:
Visible spectrum of Star #4 = J205551.3+435225. Note the
appearance of a strong HeII line at
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The photometric and spectroscopic characteristics of the third early-type
star, #4 (hereafter referred to as J205551.3+435225, after its
sexagesimal denomination in the 2MASS catalog,
2MASS J205551.25+435224.6), make it an ideal candidate
ionizing star of the North America/Pelican complex. The star is
bright in the infrared, and was included by Bally &
Scoville (1980) in the list of sources with
near-infrared characteristics consistent with an early spectral
type as their source #10. Assuming an intrinsic infrared color
characteristic of a mid O-type star, the 2MASS
color
implies an extinction
,
or
.
The extinction can also be
estimated from the B magnitude obtained from the USNO catalog,
where the star is listed as having B = 15.5, R = 11.7.
Adopting
(B-K) = -1.23 and a normal extinction law in the
visible with
R = AV / E[B-V] = 3.1 we obtain
,
which we take to be more reliable due to the wider baseline in
wavelength. The agreement with the value derived from the infrared
color is good, taking into account that the estimated extinction
is potentially affected by uncertainties stemming from the
assumption of a normal extinction law (especially in the visible,
due to the increasing dependency on grain properties as one moves
to shorter wavelengths) and from possible differences between
assumed and actual intrinsic stellar colors. The value is also in
good agreement with the extinction derived by Cambrésy et al. (2002) in that direction. The featurelesness of
the infrared spectrum (Fig. 7) indicates a spectral
type either no later than early B or later than early F, but the
Q = 0.06 derived from the
photometry strongly favors the
early option.
The resolution and signal-to-noise ratio of our infrared spectrum
does not allow us to further refine its classification (Hanson et al. 1996; Meyer et al. 1998). However, the
moderate reddening makes J205551.3+435225 accessible to visible
spectroscopy in the blue, where far more accurate classification
is possible. Besides H
and H
,
the spectrum
(Fig. 8) displays prominent HeII Pickering and
Fowler lines at
Å, 4542 Å, and 4686 Å,
which leave no doubt concerning the classification of
J205551.3+435225 as a O-type star. Also easily identifiable are
the interstellar features at
Å, 4727 Å (not
to be mistaken by HeI at
Å, which is not
visible in our spectrum), and 4762 Å with an intensity similar
to that observed in other considerably obscured O-type stars
(Hanson 2003). In contrast with the prominence of the
HeII lines, there is a nearly total absence of HeI lines. In
particular the most intense HeI in this interval, at
Å, is barely visible. Its intensity ratio with respect to
the nearby HeII line at 4542 Å, which is a sensitive spectral
subtype indicator in this range (Walborn &
Fitzpatrick 1990), clearly indicates that
J205551.3+435225 cannot be later than spectral type O5. On the
other hand, the absence of NV lines and the ratio of the two HeII
lines at 4686 Å and 4542 Å, both of which are luminosity
class indicators in the mid-O range (Walborn &
Fitzpatrick 1990), rule out a luminosity class I or III. We thus classify J205551.3+435225 as a O5V star, but note
that an even earlier spectral subclass is not excluded.
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Figure 9:
A detailed visible view of the L935 dark cloud separating the
North America and the Pelican nebulae, marking the position of
Star #4 = J205551.3+435225. The upper panel shows a
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Adopting the O5V spectral type for J205551.3+435225, and taking average absolute magnitudes and colors for stars of this type from the compilations of Drilling & Landolt (2000) and Tokunaga (2000), we obtain a distance of 610 pc to this star, in excellent agreement with the most recent determinations of the distance to the North America/Pelican complex. Moreover, the position of the star (shown in Fig. 9) is only 6'2 away from the location proposed by Matthews & Goss (1980) from their radio continuum maps, and barely outside the 5' uncertainty radius suggested by them. The ridges, and their preferential orientation facing the star, are well visible at the 1' resolution of the Canadian Galactic Plane Survey 1420 MHz radio continuum map displayed in the lower panel of Fig. 9.
The O5V spectral type and 610 pc distance also compare favorably with
the bulk properties of the complex. Its approximate angular
diameter of
corresponds to a physical diameter of 32 pc.
The electron density
can be inferred from the optically thin
radio continuum flux and the physical size by relating the
emission measure and the brightness temperature, under the
assumption of a uniform density and the adoption of a given
electron temperature (Wendker 1968). The uniform
density distribution seems to be a good approximation given the
remarkable lack of small-scale structure in the region, pointed
out by Wendker et al. (1983). Correcting
Wendker's (1968) calculation for the updated distance
of 610 pc, we obtain
cm-3. A similar value,
cm-3, is obtained using the expression given by
Goudis (1976) for that same distance and correcting for
the smaller angular size adopted in that work. Assuming the HII
region to be ionization-bounded, as suggested by the molecular
shell that surrounds it, the required ionizing flux is
s-1, which is approximately one third of
the modeled ionizing flux of a O5V star (Schaerer & de
Koter 1997). The agreement can be considered as good
given the simplified modeling of the nebula as a homogeneous
ionized sphere, although the factor-of-three excess of ionizing
photons produced by the star hints at the complex being at least
partly density-bounded.
The X-ray source 1RXS J205549.4+435216, faintly detected in the
ROSAT All-Sky Survey (Voges et al. 1999), has a nominal
position only 22'' away from our proposed ionizing star. The
offset is somewhat larger than the quoted uncertainty of 14'' in
the X-ray position, but we still consider it possible that the
X-ray source is the counterpart of J205551.3+435225. Its very high
X-ray hardness ratio,
,
can be well
explained by the large foreground extinction that we derive from
the infrared and visible colors, which absorbs the bulk of the
soft X-ray emission from the star. Assuming the temperature TXof the X-ray emitting gas to be
k TX = 0.6 - 0.7 keV, as is
typical of mid O-type stars (Berghöfer et al. 1996), we can estimate the conversion factor
between ROSAT PSPC count rate and unabsorbed X-ray flux as
erg count-1 cm-2. We note however that
this conversion factor, which we obtain adopting an extinction
AV = 3.3 E(B-V) = 8.2 (where E(B-V) is the color excess) and
a column density
cm-2 E(B-V)(Berghöfer et al. 1996), involves an extrapolation
of Fig. 3 of Berghöfer et al. (1996) to
in the steepest part of the relevant
curves, and is thus uncertain perhaps by as much as 50%. The
tabulated ROSAT PSPC count rate of
counts s-1 implies a X-ray luminosity
erg cm-2 s-1 at the distance of 600 pc.
Assuming a typical luminosity for a O5V star
(Drilling &
Landolt 2000), this yields
,
which is in fairly good agreement with the average
properties of luminous O-type stars (Berghöfer et al. 1996).
We finally note that J205551.3+435225 is remarkably isolated, as
implied by the fact that our selection criteria would have picked
up any other star earlier than B2V existing within the search
radius. If the cross-section (85 pc2 at the distance of 600 pc)
sampled a volume large enough to contain a typical representation
of the initial mass function (IMF), one could estimate the
expected number of stars with spectral types between B2V and O5V
from its ratio with respect to the number of stars O5V and earlier
expected from the IMF:
We have presented a search for the ionizing star of the North America
and Pelican nebulae in a large area ( radius) that
encompasses the geometric center of the complex and most of the
area of the L935 dark nebula that is suspected to obscure the
star. We have considered the stars whose apparent
magnitude
and
color are consistent with them being O-type stars up
to a maximum distance of 1 kpc, and used an additional
color-color criterion that excludes most late-type, reddened
stars. We have defined in this way a preliminary sample of 19
possible candidates for which low-resolution H and K band
spectroscopy has been obtained. Most of these spectra correspond
to cool photospheres, mainly belonging to AGB and carbon stars, as
expected from the infrared photometric properties of those
objects. However, three of our objects do display spectroscopic
characteristics of early- or mid-type stars. Two of them are ruled
out as possible ionizing stars, one being a foreground F5V star
and the other a background early A-type giant as confirmed by its
spectrum in the visible.
The third and brightest early-type object, J205551.3+435225, has both
photometric and spectroscopic characteristics in the infrared
consistent with it being a O5V star at the commonly accepted
distance of the complex, 600 pc, obscured by
.
We
have been able to reliably confirm this spectral classification by
means of a spectrum in the blue visible region, which excludes a
spectral type O6 or later. The properties of a nearby
ROSAT-detected source that we tentatively identify as the X-ray
counterpart of J205551.3+435225 are also consistent with those of
a typical O5V star. The location of this star, just East of the
"Florida Peninsula'' region of the North America nebula, is near
the geometric center of the complex and very close to the
accurately predicted position made from ridge structures observed
in radio continuum. The fact that no other stars with spectral
types B2V or earlier are found within
of
J205551.3+435225 indicates that this star does not highlight the
existence of a cluster or a region of enhanced stellar density.
The spectral classification of J205551.3+435225 as a O5V star, its close proximity to the expected location, the good match between its 2MASS photometry and the characteristics expected in a O5V star at the distance of the complex, and the fact that it is the only candidate fulfilling all these conditions despite the large area over which our search has been conducted, lead us to consider it very likely that the long history of the search for the ionizing star of two of the best known nebulae has been finally completed.
Acknowledgements
We are very pleased to thank the staff of the Calar Alto observatory, and especially Mr. Santos Pedraz, Mr. Alberto Aguirre, Ms. Ana Guijarro, and Mr. Jesús Aceituno, for providing us once again with excellent support during our observing run in Calar Alto. We also appreciate the constructive comments of the referee, Prof. Dr. H.J. Wendker, leading to significant improvements of this paper. AP acknowledges support from the OPTICON Network. This research has made use of the SIMBAD database operated at CDS, Strasbourg, France. It also makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. We retrieved the data of the Canadian Galactic Plane Survey as Guest Users of the Canadian Astronomy Data Centre, which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada's Herzberg Institute of Astrophysics.
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Figure 1:
The region of the North America (NGC 7000) and the Pelican
(IC 5070) nebulae with the ![]() |
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