A&A 412, 175-184 (2003)
DOI: 10.1051/0004-6361:20031417
M. S. N. Kumar1 - A. J. L. Fernandes1,2 - T. R. Hunter3 - C. J. Davis4 - S. Kurtz5
1 - Centro de Astrofísica da Universidade do Porto, Rua
das Estrelas, 7150-462 Porto, Portugal
2 - Instituto Superior da Maia, Av. Carlos Oliveira Campos, 4475-690
Avioso S.Pedro, Portugal
3 - Harvard Smithsonian Center for
Astrophysics, 60 Garden Street, MS-78 Cambridge, MA 02138, USA
4 -
Joint Astronomy Center, 660 N. A'ohoku Place, University
Park, Hilo, HI 96720, USA
5 - Instituto de Astronomia, UNAM-Morelia,
Apartado postal 3-72, CP 58090 Morelia, Michoacan, Mexico
Received 23 April 2003 / Accepted 2 September 2003
Abstract
A multi-wavelength study of IRAS 07427-2400 in line and
continuum emission was conducted to investigate the nature of a H2v=1-0 S(1) line emitting feature around this ultra-compact HII
region. High resolution 3.6 cm continuum observations from the Very
Large Array and 350 m continuum observations from the Caltech
Submillimeter Observatory, combined with archival far-infrared data of
IRAS 07427-2400 show a flux density distribution indicating a
luminous (
)
point source associated
with an ultra-compact HII region. A Grey body model fit to the flux
density distribution yields a dust emissivity index
(
)
indicative of a circumstellar disk/envelope. Our
C18O map shows a dense core centered on the continuum source,
with the major axis roughly aligned with the H2 feature. A
position-velocity diagram of the C18O core obtained along the
major axis shows rotation with a velocity gradient of
0.1 km s-1 arcsec-1. New CO J=3-2 maps of the
region are presented which reveal a massive molecular outflow from the
IRAS source. We argue that the H2 feature arises in a
disk/envelope around IRAS 07427-2400 and not in an outflow. We
present a near-infrared HK band spectrum of the H2 features that
shows several ro-vibrational emission lines of H2 and [FeII].
Analysis of the line ratios indicates that the line emission is
shock-excited and not due to fluorescence. We estimate an excitation
temperature of
1600 K and an average extinction of
mag to the H2 feature. The line fluxes yield a mass
accretion rate of
yr-1 and a lifetime of
5360
1200 yr resulting in a disk/envelope mass of
140
50
.
The resulting Jeans Mass of 2420
indicates that the disk/envelope will not undergo fragmentation.
IRAS 07427-2400 represents one of the most massive YSOs known to date
forming by means of accretion.
Key words: stars: formation - accretion: accretion disks - interstellar medium: jets and outflows - ISM: HII regions
Circumstellar disks are known to exist around low mass young stellar objects (YSOs) (see Beckwith 1999; Beckwith & Sargent 1996). Disks around low mass stars have mostly been detected by imaging the continuum emission from warm dust (Dutrey et al. 1996) or in the visible as extincted patches seen in silhouette against background light (Padgett et al. 1999; Bally et al. 2000). Observational studies of circumstellar disks around YSOs provide important evidence supporting the accretion-driven mechanism of star formation (see Shu et al. 2000). If stars of all masses form in the same way, through an accretion mechanism, disks should exist around massive YSOs and brown dwarfs in much the same way they do around low mass YSOs. Recently, the existence of disks around brown dwarfs has been demonstrated using indirect methods such as color-color diagrams (Muench et al. 2001). In a few cases of massive protostars, disks have been shown to exist through observations of molecular lines (Cesaroni et al. 1999; Shepherd et al. 2001; Zhang et al. 2001).
A class of objects known as Infrared Companions (IRC) to T Tauri stars are found to show a near-infrared excess due to on-going accretion (see Herbst et al. 1995; Koresko et al. 1997). In particular, IRCs to T Tauri, UY Aurigae and Haro 6-10 were found to emit intense H2 emission which is attributed to energetic phenomena associated with accretion shocks and magnetic fields (Koresko et al. 1997). Recently, Kumar et al. (2002, hereafter KBD02) showed the presence of rings and disk-like structures visible in H2 emission around massive YSOs. These structures may also be explained by accretion shocks and magnetic fields, especially when one considers the much higher density of material and gravitational potential around a massive star. To further investigate this possibility we present here a systematic study of IRAS 07427-2400.
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Figure 1:
Continuum subtracted H2 2.121 ![]() ![]() ![]() |
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We present near-infrared spectroscopic observations obtained with the
UIST instrument at UKIRT in December 2002. The 4 pixels wide, 120
pixels long slit was positioned along the main H2 emission features
at a Position Angle
as shown in Fig. 1.
The spectra were obtained with a plate scale of 0.12''/pixel, with
simultaneous H and K-band coverage. The observed lines are unresolved
with a spectral resolution
or
km s-1. Standard data reduction techniques were
employed including dark-subtraction and flat-field division.
Wavelength calibration was done with argon arc spectra and flux
calibration and correction for atmospheric absorption via division by
a standard star spectrum.
3.6-cm continuum emission maps were obtained using the Very Large
Array (VLA) in its B configuration. The beam size was
1.8
and the observations were sensitive
to structures up to 15
.
350
m continuum emission and
CO J=3-2 and C18O J=2-1 line emission were mapped using the
Caltech Submillimeter Observatory 10.4 m telescope at Mauna Kea,
Hawaii. 350
m emission was mapped using the SHARC bolometer
array with a pixel size of 5
.
The secondary chopper throw
was 86
in azimuth at 4.1 Hz. Calibration was based on scans
of Uranus with an airmass correction (the measured opacity in the
filter band was 0.63). Images were restored and smoothed to a
12
beam. CO J=3-2 maps were obtained with a grid size of
10
and C18O J=2-1 maps with a grid size of 15
.
The standard facility SIS Heterodyne receivers and acousto-optical
spectrometers were utilized.
Table 1: Infrared flux densities of IRAS 07427-2400.
IRAS 07427-2400 is a far-infrared (FIR) point source (d=6.4 kpc)
with a luminosity of
suggesting an O8.5 spectral type zero-age main-sequence (ZAMS) star (Panagia
1973). It is known to be associated with OH masers
(MacLeod 1991; Smits 1994; Slysh et al. 1997) and H2O masers
(Henning et al. 1992). KBD02 discovered a disk-like
structure in H2 emission around this FIR source. In Fig. 1 we
show continuum subtracted H2 v=1-0 S (1) 2.122
m emission
features (thin contours) around IRAS 07427-2400. Thick-lines along
with greyshades are used to overlay the VLA 3.6 cm continuum emission
in the same region. The star symbol shows the center of the IRAS
error ellipse and the filled circle shows the position of a 2
m
continuum source visible in the H2 narrow-band image. The straight
line at PA 107.5
marks the position of the slit used to obtain
the near-infrared HK spectrum shown in Fig. 7. This line also
indicates the axis of the bipolar H2 feature. The astrometry of
the VLA data and the H2 image are accurate to better than
1.5
and are the most precise among all the available data on
this source. These images also represent the highest spatial
resolution data available for this target. The radio source is
unresolved by the VLA beam, indicating a source size smaller than
0.3
.
This is in good agreement with the
6 cm data of Hughes & MacLeod (1993) who report the source as
unresolved at 0.3
resolution. It can be seen from Fig. 1
that the VLA continuum source, FIR source and the 2
m source are
all situated centrally (within positional uncertainties of
<1.5
)
to the bipolar H2 emission feature. Accounting
for astrometric errors, the unresolved VLA source, the IRAS point
source and the 2
m star all represent the same object, namely
IRAS 07427-2400.
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Figure 2:
Flux density distribution of IRAS 07427-2400 with a family
of one-component greybody models. As listed in Table 1, solid squares
are from IRAS, open squares are from MSX and the triangle is from
SHARC (this paper). The solid line marks the best fit to the 60, 100
and 350 micron flux densities (with zero degrees of freedom). The
eight dashed lines describe models which pass through the 1-sigma
uncertainties of the flux densities. The corresponding range of
fitted values and derived physical properties are listed in Table 2.
The curve that comes closest to the mid-infrared data is the one most
like a blackbody (i.e.
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A summary of the flux density measurements for this region (along with
the corresponding beamsizes) is given in Table 1. These values
(except for the FIRSSE values) are plotted in Fig. 2, along with a
family of greybody models. The infrared through sub-millimeter flux
densities cannot be fit by a single temperature greybody spectrum. In
lieu of a radiative transfer model, we have chosen to fit only the
cold component of the emission (60-350 microns), as it contains the
bulk of the source luminosity. Since the chopper throw of the
350 m observations is larger than the fitted size of the IRAS
60
m data (both the HIRES beam and source image), we have
assigned all of the 60 and 100
m emission to the 350
m
source. A complete grid of models was calculated with the parameters
being temperature (T), dust emissivity index (
)
and optical
depth (
). The best-fit model has T = 46 K,
and
.
To estimate the
uncertainty of the results, we also fit models to each combination of
the 1-sigma uncertainties in the three measured flux densities. The
full range of fitted values and the derived physical quantities are
listed in Table 2. The value for
is not well-constrained,
mainly due to the lack of a millimeter wave flux measurement. The
models range from 2-14 Jy in their prediction at 1.3 mm, thus a
future measurement there would be useful. Nevertheless,
appears to be significantly less than the typical value of 2 seen in
interstellar gas (Hildebrand 1983) and is more consistent
with grain growth (or high optical depth) as seen toward the central
peak of low-mass protostellar objects (e.g. Hogerheijde & Sandell
2000). In any case, the range of values for the luminosity and
mass of the dust emission are consistent with the presence of a deeply
embedded massive young stellar object.
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Figure 3:
HIRES processed IRAS 12 ![]() ![]() |
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Table 2: Derived physical properties of IRAS 07427-2400.
In Fig. 3 we show the 350 m continuum emission, C18O emission map and HIRES processed IRAS 12
m map overlaid on the
H2 image. These tracers arise from the dense core of a star
forming region. The 350
m emission that largely traces the cold
dust in dense cores is found to be uniformly distributed on
IRAS 07427-2400 and also encloses the H2 emission feature. The
C18O emission is concentrated in a central ellipse of
20
(
). The major
axis of the C18O core is roughly aligned with that of the H2feature. A position-velocity (PV) diagram along this axis is shown in
Fig. 4. It can be seen that the eastern and western edges of the
C18O core are doppler shifted to red and blue respectively from
km s-1, implying a rotating core. Further, it can
be seen that the velocity spread at the center of the core is larger
(
5 km s-1) compared to that at the edges
(
2 km s-1), implying that the center of the core is
rotating much faster than the edges. A straight line fit to the PVdiagram results in a velocity gradient of
0.1 km s-1 arcsec-1 and indicates approximately
Keplerian rotation. The area averaged C18O line widths are
3.4 km s-1. Note that these velocities are similar to the
line widths of H13CO+ profiles (
3 km s-1) measured
by Cesaroni et al. (1999) in the case of a disk around
IRAS 20126+4104, a prototypical massive protostar. The
continuum emission and the C18O data together show that the H2 feature arises in the plane of a rotating disk/envelope surrounding
IRAS 07427-2400.
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Figure 4:
Position-velocity diagram of the C18O core along the major
axis aligned East-West. The emission is summed over a width of
45
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Figures 5 and 6 show the bipolar molecular outflows around
IRAS 07427-2400. In Fig. 5 we reproduce the CO J=1-0 contour map
from Shepherd & Churchwell (1996, hereafter SC96) overlaid on
the H2 grey scale image from KBD02. The CO J=1-0 outflow is
well-centered on IRAS 07427-2400, shown by a star symbol. The PA of
the outflow axis is between 20
and 30
as measured from
Fig. 6 of SC96. These authors argue that the CO J=1-0 emission
traces a massive molecular outflow from IRAS 07427-2400. They
estimate an outflow mass of 8.3
,
a momentum of
93
km s-1, and a timescale of
yr, resulting in a mechanical luminosity of
3.9
.
This is an order of magnitude higher than that
of L1551 IRS5, a prototypical outflow from a low mass protostar
(Fridlund et al. 1989).
In Fig. 6 we present new maps of CO J=3-2 integrated high velocity
emission overlayed on to the grey scale H2 image. Figures 6a and
6b show channel maps that separate low and high velocity components.
The velocity shifts are with respect to
km s-1.
The CO J=3-2 emission shows a prominent bipolar outflow at
and a weaker component at
.
The
weaker component is at relatively lower velocities of
6-11 km s-1 while the main flow has velocity components
extending up to
17 km s-1. We estimate a total mass of
36
,
momentum of 253
km s-1 and an
outflow timescale of
yr, resulting in a CO
mechanical luminosity of
1
.
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Figure 5:
CO J=1-0 high velocity emission contours from SC96 overlaid
on a continuum subtracted H2 image at 2.122 ![]() |
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Figure 6:
CO J=3-2 high velocity emission contours overlayed on a
continuum subtracted H2 grey scale image. a), b) Channel
maps in velocity intervals (w.r.t
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It appears from Figs. 5 and 6 that there are at least two massive
outflows in the region at different position angles. The first
question that arises is whether the flows traced by CO J=1-0 and CO J=3-2 are really different. The half power beam width of the CO J=1-0 map is 60
while that of the CO J=3-2 map is
20
.
Given the large difference in resolution and the
possibility that gas in different parts of the outflow lobes are at
different temperatures, these maps may trace the same outflow.
However, the axes of the flows differ in PA by
over an
extent of 90
,
which cannot be easily explained by differing
angular resolution. This effect could be produced if the CO J=1-0
traces a larger angular extent than CO J=3-2. Mapping the outflow in
both lines with similar angular resolution can resolve this issue. In
any case, the axes of these two outflows do not coincide well with the
axis of the H2 feature. The mean PA (
25
)
of the CO J=1-0 outflow is nearly perpendicular to the axis of the H2emitting feature (
). The PA of the high velocity CO J=3-2 outflow is also clearly about 30
away from the H2axis. Only the low velocity component at
is
spatially well-correlated with the axis of the H2 feature.
It is well-known that H2 emission and CO outflows are spatially
well-correlated (see Davis & Eislöeffel 1995;
Bachiller 1996) where H2 traces the cavities of the CO
outflow. The fainter H2 features along the North-South direction
that appear as nested bows (see KBD02) could delineate the cavity
boundaries of the CO J=1-0 emission along the red and blue lobes.
Further, H2 emission associated with outflows is known to appear as
jets/knots that generally show increasing collimation with increasing
distance from the driving source. The dominant H2 feature
resembles the morphology of a disk more than that of a jet/outflow.
Also the H2 emission is bright only near the source and abruptly
terminates beyond 5
- a behavior that is extremely unlikely
to be observed in outflows. Therefore, the H2 feature shown
in Fig. 1 is probably not a part of the massive CO outflow, but
rather arises in a disk-like structure associated with the low
velocity component of the CO J=3-2 emission.
The presence of CO outflows, C18O and dust emission at
350 m all coinciding with the H2 feature indicates the
existence of dense, cold, neutral material around the H2 feature.
Therefore, the physical condition producing the H2 emission is also
expected to show its signatures in the molecular lines of CO which is
more easily excited than H2. However, such a detection strongly
depends on the beam filling factor of the feature and the
signal-to-noise ratio. The H2 emitting region is much smaller than
the CO emitting region so the beam filling factor is expected to be
very low, resulting in difficulties to identify the feature in CO
lines. The feature will probably show up well in high Jsub-millimeter CO lines (e.g., 7-6) that have upper level energies
>150 K, and perhaps even lower J lines with the use of
interferometers.
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Figure 7:
Near-infrared H and K-band spectra of the H2 emitting
feature shown in Fig. 1. The top panel displays the spectrum of the
middle 1.2
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In this section we present the analysis of a near-infrared H & K band
spectrum obtained on the putative disk-like H2 feature, in order to
evaluate the H2 excitation mechanism. The near-infrared H & Kband spectrum was obtained with the slit placed along the equatorial
axis of the possible star-disk system as shown in Fig. 1. Two spatial
cuts along the slit were extracted to identify the emission from
STAR and DISK. The continuum strip on the spectral
images that fell on the middle 10 pixels enclosing the 2 m
visible star (width of 1.2
)
was extracted as the
STAR spectrum which likely includes contributions from the
unresolved UCHII region. The remaining parts of the spectrum that were
visible only in emission lines and associated with the extended H2feature were summed (over 70 pixels) to obtain the DISK
spectrum. In Fig. 7 we present these two spectra
separately with the STAR spectrum displayed in the top panel
and the DISK spectrum displayed in the bottom panel. The
most prominent features in the K band are the H2 emission lines
arising from several ro-vibrational transitions. In the H band we
have detected the a4D7/2-a4F9/2 1.644
m and the
a4D7/2-a4F7/2 1.810
m [FeII] lines and the
v=1-0 S (6) and S (7) H2 emission lines. Unlike the DISK
spectrum, the STAR spectrum includes strong detections of
Brackett 10 in the H band plus Br
in the K band. Comparing
the K window spectra at both positions, we noticed an unusually bright
v=2-1 S (5) H2 line appearing in the STAR spectrum when
compared to the other v=2-1 line fluxes. For example, the 2-1 S (5)/S (1) line ratio for the DISK spectrum is
0.8
0.2
whereas for the STAR this ratio is
2.8
0.8. This
apparent enhanced emission from the high vib-rotational v=2, J=8 H2level may arise due to more energetic conditions close to the star.
However, the atmospheric transmission near the 1.95
m lower end of
the K window makes the line fluxes very uncertain. For this reason,
in the following analysis we do not use the 2-1 S (5) and the 1-0 S
(3) lines, but rely upon the remaining lines. Thus, we computed line
fluxes for all the remaining H2 lines detected in the spectra by
fitting single or multiple Gaussian profiles to account for line
blending.
The amount of reddening due to foreground extinction can be estimated
by using a pair of H2 emission lines arising from a common upper
level. The intensity ratio of such lines is uniquely determined by
known physical parameters and the extinction value and does not depend
on the excitation mechanism (see e.g., Fernandes et al.
1997). The near-infrared K-band contains several rotational
transitions of the H2 v=1 to v=0 emission line pairs such as [S (1), Q (3)] and [S (0), Q (2)]. In practice, only the strongest
observed line pair is used since the Q-lines are present at the edge
of the K-band atmospheric window, which precludes an accurate
determination of the absolute line fluxes. Therefore, we have used
the [S (1), Q (3)] line pair to estimate the differential extinction
from 2.121 to 2.406 m. The extinction at any other wavelength
can then be determined by adopting the extinction law:
valid for the near-IR wavelengths
(Draine 1989). The visual extinction can then be
determined by assuming
.
The estimated visual
extinction found for the (STAR+DISK) emission is
mag. We found that the inner STAR region
(
)
is more extincted than the outer DISK
region (
)
by
mag. The dust emission
model column density range (see Table 2) predicts 15-33 mag of Avassuming that the source is placed at the center of the cloud (and
using a conversion factor of
cm-2 per mag).
This range actually contains the Av predicted above from the
near-infrared spectrum.
Conversion of all the intrinsic fluxes to column densities of the
upper transition levels provides a tool to inspect the H2population and thus infer the gas excitation conditions. Figure 8
shows the derived column density diagram of the observed H2 lines
for the total (STAR+DISK) emission. If the shocked gas is
relaxed at some temperature then the H2 levels will be populated
according to a Boltzmann distribution and in Fig. 8 the data points
should fall in a straight line where the slope is inversely
proportional to the excitation temperature of the gas. In Fig. 8 we
show the v=1 upper level data points by squares and v=2 upper level
data points by triangles along with their corresponding error bars. A
linear fit to the data indicates an excitation temperature of
K for the H2 emission feature. Filled circles in
Fig. 8 show the predicted positions for fluorescent excited H2population according to the models of Black & van Dishoeck
(1987). Shock and fluorescent excitation can be discerned
clearly only by observations of emission lines with upper energy
levels of v=2 or higher. Our data shows only emission lines
originating from v=1 and v=2 upper levels. Nevertheless, the model
predictions of fluorescent excitation (for v=2 levels) clearly lie
above the observed points (shown by triangles) indicating that the
observations do not support fluorescent excitation. Thus the observed
H2 emission is understood to be originating from shock waves.
Separate diagrams made for each region show remarkable similarity of
the H2 excitation temperature along the observed emission axis.
Within the errors, the excitation temperature derived for the
DISK region of
K is similar to the value
obtained for the STAR region of
K. The presence
of strong [FeII] lines is expected in fast discontinuous shocks
(J-shocks) where shock velocities exceed 50 km s-1 (Hollenbach
& McKee 1989). J-shocks would completely dissociate H2,
however, so either H2 reforms in a region downstream from the
supersonic shock front or the H2 is excited in weaker continuous
shocks (C-shocks) that are softened by magnetic fields closer to the
central source.
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Figure 8: Diagnostic diagram showing the dereddened H2 column densities versus the upper level energy in Kelvin. The diagram plots the natural logarithm of the column density divided by the statistical weight, against the upper energy level for each line transition. The plotted lines arise from vibrational levels v=1 (squares) and v=2(triangles). The straight line represents the best linear fit through the observed data points. Filled circles are predictions from Black & van Dishoeck (1987) for fluorescent excitation. |
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The STAR spectrum displayed in the top panel of Fig. 7 shows
Br
emission. This emission is an average measured within 1.2
of the 2
m star. Since the seeing was less than 1.2
we expect that the STAR essentially encloses all
the contribution from the massive young star and the UCHII region
which is shown to be smaller than 0.3
(see Sect. 3.1). Using
line fits to the spectrum, we measured a flux of
W cm-2 arcsec-2. After
correcting for the extinction in the central region
(
mag), we obtain a Br
flux of
W cm-2 arcsec-2. The
Br
flux predicted for a 100 km s-1 J-shock with a
number density of 105 cm-3 is about
W cm-2 arcsec-2 (see Burton et al. 1989). This is in close agreement with the measured
flux for the STAR spectrum implying that the observed
Br
flux can arise in fast J-shocks anywhere within the inner
6000 AU envelope to the star; although contributions from an UCHII
region cannot be ruled out. In comparison, a much higher level of
Br
flux is observed in the DR21 outflow where fluorescent
excited H2 emission is detected (Fernandes et al. 1997). The presence of [FeII] emission in the
DISK spectrum indicating shocks up to 50 km s-1 in the
outer envelope and Br
in the STAR spectrum
indicating shocks up to 100 km s-1 in the inner envelope thus
strongly support that the envelope as a whole is dominated by shocks
rather than fluorescence.
The central placement of the continuum sources and the UCHII region
over the bipolar H2 feature of Fig. 1 shows that the FIR source
IRAS 07427-2400 is the center of this emission. The uniform
distribution of the C18O and 350 m emission over the IRAS
source and the H2 feature indicates a dense core that encloses
them. The grain emissivity index
set as a free parameter in
our grey-body model fits indicates a value
0.66 which is
much less than the value found in the average interstellar medium
where
2. Indeed, it is similar to that of a circumstellar
disk such as HL Tau (
,
see Beckwith &
Sargent 1991). It is now fairly well established that
circumstellar disks have
due to the growth of particle
sizes (Beckwith et al. 2000). This is true even for large
circumstellar envelopes of few thousand AU radii (Hogerheijde &
Sandell 2000). Therefore the observed value of
in
IRAS 07427-2400 indicates the presence of a circumstellar
disk/envelope structure whose dimensions are larger than the H2emission feature.
As shown in Sect. 3.2 the H2 feature arises not in the plane of
the outflow axis but in a nearly perpendicular plane. The fainter
component of CO J=3-2 at low velocities of 6-11 km s-1 at
matches closely with the axis of the H2 feature.
Thus we infer that most of the slow moving gas in the envelope is
restricted to this plane which is also the major axis of the C18O
core. Since the C18O core shows evidence for Keplerian rotation,
we can compute the net mass enclosed within the radius of the observed
core. For a spherical envelope of gravitating mass in virial
equilibrium,
v2 = GM/R, where M is the mass of the material
moving with velocity v inside a radius R. The velocity gradient
of 0.1 km s-1 arcsec-1 derived from the PV diagram and the
projected radius R=0.62 pc of the C18O core implies a velocity
v=2 km s-1 and an enclosed mass of
.
Note
that the single component grey-body fit (Fig. 2) predicts a source
mass of
400
(see Table 2). While the source can
account for two-thirds of the above mass, the remaining one-third of
200
can be attributed to the mass of a surrounding
torus. These pieces of evidence suggest that the H2 emitting
feature arises from a huge rotating torus in the disk plane. Such
tori/envelopes are known to exist in other massive YSOs (Shepherd &
Kurtz 1999; Cesaroni et al. 1999).
As shown in Sect. 4.2 the line ratios indicate the purely shocked nature
of the H2 emission. Emission lines that can be attributed to the
presence of intense ultra-violet light are found only within the
central 1.2
enclosing the STAR and the UCHII region.
Both Br
and Brackett 10 are bright only in the STAR
spectrum. The compact nature of the HII region
(
0.3
)
indicates that the ionizing
radiation from the central source influences only the inner 1000 AU
radius and is unable to affect the larger
33 000 AU structure.
The H2 gas is found to be in equilibrium at a temperature of
1600 K.
There is still much controversy about the nature of shocks occurring
in molecular cloud cores. C-type shocks, strongly dependent on a
significant ambient magnetic field entrained in a low ionization gas
cloud, have been modeled to produce significant column densities of
warm H2 gas at gas temperatures below 16 000 K for
cm-3 or below 4000 K for
cm-3 (Le
Bourlot et al. 2002). On the other hand, J-type shocks
form in relatively high degree of ionization gas regions, where
magnetic fields are too weak generating a single fluid hydrodynamic
shock wave (e.g., Smith 1994). H2 can be excited in
J-shocks with velocities as low as 8-10 km s-1 and can be
observed in reformation zones when velocities reach up to
200 km s-1.
But the excitation of [FeII] requires J shocks with velocities of at
least 40-60 km s-1 for pre-shock gas densities of
104 cm-3. The presence of both H2 and [FeII] lines in the
spectrum strongly suggests the presence of J type shocks rather than C
shocks since [FeII] cannot be excited in the presence of C shocks
alone (Gredel 1994). The unreddened H2 1-0 S(1) line
intensity of
W m-2 arcsec-2 is about 3
times larger than the [FeII] 1.644
m line intensity of
W m-2 arcsec-2. Such a high value can
only be modelled with fast J-shocks of 80-100 km s-1 in gas
density of 104 cm-3 (Hollenbach & McKee 1989;
Smith 1994). Moreover, we computed the unreddened column
density of the v=1, J=3 upper level (from where the 1-0 S(1) H2line arises) to be
cm-2, a value that is
well-explained by slow, non-dissociative J-shock models
(Smith 1994). This result can be compared with HH120, where
Nisini et al. (2002) found the H2 1-0 S(1)/[FeII]
1.644
m ratio to be 4.5 which they modeled with fast J shocks. On
the other hand, C-type shocks with velocities above 20 km s-1produce much larger columns of warm gas in the v=1, J=3 excited level
of
-1019 cm-2 (Smith 1993) which are
not observed here. It thus appears that C-shocks cannot explain
the observed H2 emission.
The observed H2 spectrum also suggests that most of the gas is
shocked by low velocity non-dissociative shocks. If dissociative
shocks alone were present, H2 would dissociate and reform
behind the shock front. H2 recombines through higher vibrational
levels of v=6 (Black & van Dishoeck 1987) resulting in
brighter lines from levels of v>3. This is not seen in our spectrum
(see Fig. 7) indicating the absence of higher velocity J shocks.
Moreover, H2 reforms in gas at much lower temperatures than the
value obtained here of about
K. We conclude that J
shocks (with
)
are responsible for the H2 excitation.
This implies that magnetic fields may not be important in shaping
the H2 disk/envelope.
In Sect. 4.1 we showed that the estimated visual extinction produces a
difference of
mag between the STAR and DISK
region. Applying a conversion factor of
cm-2 and assuming that the
projected radius of the H2 feature (
AU) induces
the difference of 5.4 mag in Av, we can compute a density
contrast between the star and the edge of the disk. The density
contrast implies a log
(
density) which is
typically expected just after the formation of the initial stellar
core in a spherical protostar (see Fig. 2 of Larson 1969).
The dimensions at which this density contrast can be expected
(
1017 cm = 10 000 AU) are consistent with the observed size
of the H2 feature.
We now evaluate the approximate disk/envelope mass, a mass accretion
rate and their implications. Based on the narrow-band image and the
HK spectrum analysis, we assume an average value of H2 v=1-0 S(1)
line flux (
W m-2 arcsec-2)
spread over a disk of angular area
26 arcsec2. Using the
relation
d2 (Davis &
Eislöeffel 1995) we obtain
,
where we have assumed a distance of 6.4 kpc. The estimated extinction
(
mag) yields a corrected H2 luminosity of
.
It is possible to calculate an approximate mass flow rate and a
related net H2 mass using this luminosity. Such a calculation
critically depends upon the assumed velocity of the H2 gas within
the disk. If we use an average velocity of 30 km s-1,
,
we obtain a mass accretion rate
yr-1for which we used a value of
(Smith 1995).
This accretion rate is about 25 times larger than that for
IRAS 20126+4104 where
yr-1(Cesaroni et al. 1999). This result may not be surprising
given that the FIR luminosity of this source is about 5 times higher
than that of IRAS 20126+4104. The disk lifetime
yr combined with the mass accretion rate yields a
disk mass of
suggesting that nearly
one quarter of the total mass (protostellar envelope = 625
)
is
in the form of disk material.
The results and analysis presented above show a self-consistent picture of a massive YSO associated with an UCHII region, a massive outflow and a massive rotating disk/envelope. IRAS 07427-2400 is a more massive and evolved object than the well-known massive protostar IRAS 20126+4104 (Cesaroni et al. 1999). The luminosity of the star derived from the grey-body model fits indicates an O9-O8.5 type ZAMS star (Panagia 1973) . Despite the ambiguities discussed in Sect. 3.2 the CO J=1-0 and CO J=3-2 emission could be tracing a single massive bipolar outflow. Higher spatial resolution observations will be necessary to sort the issue of multiple flows if any. The observations presented here demonstrate for the first time, a massive rotating disk/envelope visible in H2 emission.
The estimated mass accretion rate and disk lifetime strongly support
the scenario of massive star formation by accretion. The interesting
question in this case is the future of the mass in the disk/envelope.
The estimated disk mass of
means that
there is sufficient material to form several tens to a hundred low
mass stars. Further, the disk is hot at 1600 K and possibly clumpy in
nature. The range of velocities from 2 km s-1 to
40-60 km s-1 within the disk feature could be explained by
assuming a clumpy disk. However, the question is whether this
disk/envelope will undergo fragmentation to form several stars or if
the material will continue to feed onto the central massive star. We
compute the Jeans Mass (p. 303, Lang 1999) using the
estimated temperature (1600 K) and an average mass density
gm cm-3 based on the 350
m
observations and H2 observations. The result is
.
This implies that a warm
disk/envelope such as this cannot undergo fragmentation due to
insufficient mass.
Given the presence of an UCHII region, it is tempting to think that
the central massive star has formed a stable hydrostatic core. The
central UCHII region is expected to expand and has the potential to
evaporate the disk material. The expansion rate of the UCHII region
G5.89-0.39 was measured to be 35 km s-1 by Acord et al. (1998). If we assume this expansion rate, 5000 yrs
(
disk life time) is required for the present epoch UCHII region
to expand to the size of the H2 disk. Alternatively if we just
assume a 10 km s-1 ionized gas sound speed for the expansion
velocity, the expansion time will be 16 000 yr (
life time). In any event it is most likely that the UCHII region will
expand and evaporate the disk instead of the disk fragmenting or
feeding the central star. In view of these results, the ring
structures visible in H2 emission around massive YSOs (KBD02, De
Buizer 2003) may indicate remnants of such an evaporated disk,
or cases where a small portion of the envelope is yet to be
evaporated. While the radii of the H2 emitting features seem too
large for a proper accretion disk, a gravitationally bound, flattened
structure of this nature is not improbable in view of the observations
and arguments presented above. Indeed, even in the case of low mass
stars, huge (2000-3000 AU) envelopes that appear as flattened
structures are known to exist (Kumar et al. 1999;
Weintraub et al. 1994).
We present multiwavelength line and continuum observations around a
putative disk structure visible in the shock excited H2 line
emission at 2.122 m.
1) The observations presented here support a scenario for the presence
of a massive rotating disk/envelope around the luminous YSO
IRAS 07427-2400 associated with an UCHII region. Its association with
a massive bipolar outflow and a massive disk/envelope makes this one
of the most massive YSOs known to date forming via accretion. The
disk-like feature seen in H2 emission is found to be perpendicular
to the outflow axis and strongly coincides with dense gas tracers
indicative of circumstellar disks/envelopes. PV diagram of the
C18O map indicates a rotating core with a velocity gradient of
0.1 km s-1 arcsec-1.
2) Near-infrared H & K-band spectra of the H2 feature show
several ro-vibrational emission lines of H2, the ratios of which
indicate shock excitation of H2. We measure an excitation
temperature of 1600 K, and an average extinction of Av=11 mag
in the line emitting regions. Further, the measured H2 fluxes and
detection of [FeII] lines indicate the presence of J shocks rather
than C shocks. The detection of Br
emission close to the
central source can be explained with fast J shocks alone, although
contributions from the UCHII region cannot be ruled out.
3) Using the measured line fluxes, we estimate an envelope mass
accretion rate
yr-1 and a life time of
5400 yr resulting in a mass of
.
Computation of the Jeans Mass for the observed disk/envelope
temperature and number densities indicate that the disk/envelope can
not undergo fragmentation. High spatial and spectral resolution
observations at near-infrared wavelengths can throw much light onto
the detailed properties of the disk/envelope structure and the source
of excitation of the H2 emission.
Acknowledgements
The United Kingdom Infrared Telescope is operated by the Joint Astronomy Center on behalf of the UK Particle Physics and Astronomy Research Council. The UKIRT data reported here were obtained as part of its Service Pro gramme. Observations at the CSO are supported by NSF grant AST-9980846. This research made use of data products from the Midcourse Space Experiment. Processing of the data was funded by the Ballistic Missile Defence Organization with additional support from NASA office of Space Science. This Research has also made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We thank an anonymous referee for many useful suggestions.