Issue |
A&A
Volume 510, February 2010
|
|
---|---|---|
Article Number | A5 | |
Number of page(s) | 16 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200913215 | |
Published online | 29 January 2010 |
Three intermediate-mass young stellar
objects with different properties emerging from the same natal cloud in
IRAS 00117+6412![[*]](/icons/foot_motif.png)
Aina Palau1 - Á. Sánchez-Monge2 - G. Busquet2 - R. Estalella2 - Q. Zhang3 - P. T. P. Ho3,4 - M. T. Beltrán5 - H. Beuther6
1 - Centro de Astrobiología (INTA-CSIC), Laboratorio de Astrofísica
Estelar y Exoplanetas, LAEFF campus, PO Box 78, 28691 Villanueva de la
Cañada, Madrid, Spain
2 - Departament d'Astronomia i Meteorologia (IEEC-UB), Institut de
Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1,
08028 Barcelona, Spain
3 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138, USA
4 - Academia Sinica, Institute of Astronomy and Astrophysics, PO Box
23-141, Taipei 106, Taiwan
5 - INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125
Firenze, Italy
6 - Max-Planck-Institut for Astronomy, K
nigstuhl 17, 69117 Heidelberg,
Germany
Received 31 August 2009 / Accepted 10 November 2009
Abstract
Aims. Our main aim is to study the influence of the
initial conditions of a cloud in the intermediate/high-mass star
formation process.
Methods. We observed with the VLA, PdBI, and SMA the
centimeter and millimeter continuum, N2H+ (1-0),
and CO (2-1) emission associated with a dusty cloud harboring
a nascent cluster with intermediate-mass protostars.
Results. At centimeter wavelengths we found a strong
source, tracing a UCH II region, at the
eastern edge of the dusty cloud, with a shell-like structure, and with
the near-infrared counterpart falling in the center of the shell. This
is presumably the most massive source of the forming cluster. About
15'' to the west of the UCH II region and
well embedded in the dusty cloud, we detected a strong millimeter
source, MM1, associated with centimeter and near-infrared emission. MM1
seems to be driving a prominent high-velocity CO bipolar outflow
elongated in the northeast-southwest direction, and is embedded in a
ridge of dense gas traced by N2H+,
elongated roughly in the same direction as the outflow. We estimated
that MM1 is an intermediate-mass source in the Class 0/I phase. About
15'' to the south of MM1, and still more deeply embedded in the dusty
cloud, we detected a compact millimeter source, MM2, with neither
centimeter nor near-infrared emission, but with water maser emission.
MM2 is associated with a clump of N2H+,
whose kinematics reveal a clear velocity gradient and additionally we
found signposts of infall motions. MM2, being deeply embedded within
the dusty cloud, with an associated water maser but no hints of CO
outflow emission, is an intriguing object, presumably of intermediate
mass.
Conclusions. The UCH II
region is found at the border of a dusty cloud which is currently
undergoing active star formation. Two intermediate-mass protostars in
the dusty cloud seem to have formed after the UCH II
region and have different properties related to the outflow phenomenon.
Thus, a single cloud with similar dust emission and similar dense gas
column densities seems to be forming objects with different properties,
suggesting that the initial conditions in the cloud are not determining
all the star formation process.
Key words: stars: formation - dust, extinction - H II regions - ISM: individual objects: IRAS 00117+6412 - radio continuum: ISM
1 Introduction
It is well established that intermediate (2-8
)
and high-mass (
8
)
stars form in cluster environments (e.g., Kurtz et al. 2000; Evans
et al. 2009),
and that at the very first stages of their formation, they are
embedded within dense and cold gas and dust from their original natal
cloud. However, it is not clear to what extent the initial conditions
of the natal cloud are determining the star formation story of the
cloud and the properties of the nascent stars of low, intermediate, and
high mass, forming within it.
Regarding the star formation story, there is increasing evidence that
a single cloud can undergo different episodes of star formation, as
suggested by studies of deeply embedded clusters observed at spatial
scales similar to the cluster member separation (
5000 AU). In
these studies, the intermediate/high-mass young stellar objects
(YSOs) in the forming cluster seem to be in different evolutionary
stages (judging from the peak of their spectral energy distributions:
Beuther et al. 2007;
Palau et al. 2007a,b;
Leurini et al. 2007;
Williams et al. 2009).
Table 1: Main parameters of the VLA, PdBI, and SMA observations.
Concerning the properties of the intermediate/high-mass YSOs
forming
within the cloud, a few studies toward massive star-forming regions
have revealed that two objects formed in the same environment may have
very different properties in the ejection of matter, with one YSO
driving a highly collimated outflow nearby another YSO driving an
almost
spherical mass ejection (e.g., Torrelles et al. 2001,
2003;
Zapata et al. 2008),
indicating that
the initial conditions in the cloud may not be the only agent
determining the formation and evolution of the intermediate/high-mass
members in a forming cluster. While most of these cases have been
found at very high spatial scales (1 AU), a broad
observational
base in other regions and at other spatial scales is required to
properly understand the role of initial conditions of the cloud in the
cluster formation process and ultimately in the formation of
intermediate/high-mass stars.
In this paper we show a high angular resolution study of a
deeply
embedded cluster whose intermediate-mass protostars show differences
not only in their spectral energy distributions, but also in the
ejection phenomena associated with the YSOs.
The region, IRAS 00117+6412, was selected from the list of
Molinari et al. (1996)
in a search for deeply embedded
clusters which are luminous (>1000
)
and nearby (distance
<3 kpc).
The IRAS source has a bolometric luminosity of 1400
and is
located at a distance of 1.8 kpc (Molinari
et al. 1996).
The millimeter single-dish image reported
by Sánchez-Monge et al. (2008)
shows strong
emission with some substructure, tracing a dusty cloud, and the
centimeter emission reveals an ultra-compact H II
(UCH II) region,
associated with the brightest 2MASS source of the field at the eastern
border of the dusty cloud. The dusty cloud is associated with an
embedded cluster reported by Kumar et al. (2006), two H2O
masers spots (Cesaroni et al. 1988;
Wouterloot
et al. 1993),
and CO (2-1) bipolar outflow emission
(Zhang et al. 2005;
Kim & Kurtz 2006).
All
this is suggestive of the dusty cloud harboring a UCH II
region and a
deeply embedded stellar cluster forming in its surroundings.
We conducted high-sensitivity radio interferometric observations in order to study the different millimeter sources embedded in the dusty cloud. To properly characterize the protostars, we also studied the distribution of dense gas, outflow and ionized gas emission. In the present paper we show the first results obtained from N2H+ dense gas and CO outflow emission, focusing mainly on the intermediate/high-mass content of the protocluster.
2 Observations
2.1 Very Large Array observations
IRAS 00117+6412 was observed with the Very Large Array
(VLA) at 6 and 3.6 cm
with 5-8 EVLA
antennae in the array. The phase center of these observations was
,
and
.
The integration
time was about 45 minutes at both wavelengths. Absolute flux
calibration was achieved by observing 3C48, with an adopted flux
density of 5.48 Jy at 6 cm, and 3.15 Jy at
3.6 cm.
The data reduction followed the VLA standard guidelines for
calibration and imaging, using the NRAO package AIPS. Images were
performed using different weightings. The robust parameter of Briggs
(1995) was
set equal to 1 and 3 (almost natural)
respectively at 6 and 3.6 cm. The configuration of the array,
observing dates, phase calibrator and bootstrapped fluxes, synthesized
beams and the continuum rms noise achieved are listed in Table 1.
The observations at 7 mm (together with NH3
and SiO molecular line
observations, Busquet et al. in prep.) were carried out with 9
EVLA
antennae in the array. In order to minimize the effects of atmospheric
fluctuations, we used the technique of fast switching
(Carilli
& Holdaway 1997)
between the source and the
phase calibrator over a cycle of 120 s, with 80 s
spent on
the target and 40 s on the calibrator. The on-source
integration
time was about 1.8 h. Absolute flux and phase calibrations
were
achieved by observing 3C 286 (1.45 Jy) and 0102+584
(see Table 1). Data
reduction was performed following the VLA guidelines for the
calibration
of high frequency data, using the NRAO package AIPS. The image was
constructed using natural weighting and tapering the uv-data
at
50 k
to increase the signal-to-noise ratio.
Searching the archive of the VLA, we found B-array data at 3.6 cm and C-array data at 1.3 cm observed in 1992 (project AC295). The data were reduced with the standard AIPS procedures.
2.2 Plateau de Bure Interferometer observations
![]() |
Figure 1:
IRAS 00117+6412 continuum maps.
a) Grey-scale: 2MASS |
Open with DEXTER |
The continuum emission at 3 and 1.2 mm was observed
simultaneously
together with the N2H+ (1-0),
and CS (5-4) molecular transitions
with the Plateau de Bure Interferometer (PdBI).
The
observations were carried out during 2004 October 17 and December 7,
with the array in the D (5 antennae) and C (6 antennae)
configurations, respectively. The phase center was
,
and
,
and the projected baselines
ranged from 24.0 to 229.0 m. The system temperatures were
300 K
for the receiver at 3.2 mm, and
K
for the receiver at
1.2 mm for both days. Atmospheric phase correction based on
the 1.2 mm
total power was applied. The receiver at 3.2 mm was tuned to
93.17378 GHz (lower sideband) to cover the N2H+ (1-0)
transition,
for which we used a correlator unit of 20 MHz of bandwidth and
512
spectral channels, providing a spectral resolution of 0.04 MHz
(0.13 km s-1). The receiver at
1.2 mm was tuned to 244.93556 GHz (upper
sideband), covering the CS (5-4) transition, and the CH3OH
50-40 A+ and 5-1-4-1
E transitions. The CS and CH3OH line emission at
1.2 mm will be presented in a subsequent paper (Busquet
et al., in prep.).
For the continuum measurements we used two
correlator units of 320 MHz in each band for both receivers.
The FWHM
of the primary beam was
54''
at 3.2 mm, and
22''
at
1.2 mm.
Table 2: Multiwavelength results for the intermediate-mass YSOs in the star-forming region IRAS 00117+6412.
Bandpass calibration was performed by observing 3C454.3 for
October 17
and 2145+067 for December 7. We used the source 0212+735 to calibrate
the phases and amplitudes of the antennae for both days. The rms noise
of the phases was
for the data at 3.2 mm and <
at
1.2 mm. The absolute flux density scale, calibrated with
MWC349 on October 17
and 2145+067 on December 7, has an estimated uncertainty of
around 15%. Data were calibrated using CLIC and imaged with MAPPING,
both part of the GILDAS software package. Imaging of the 3 and
1.2 mm
emission was performed using natural and uniform weighting,
respectively. See Table 1 for details on the synthesized beams
and the
rms noises.
Table 3: Parameters of the 1.2 mm subcondensations associated with MM1 from the uniform-weighted PdBI mapa.
2.3 Submillimeter Array observations
The Submillimeter Array (SMA;
Ho et al. 2004)
in
the compact configuration was used to observe the 1.3 mm
continuum
emission and the 12CO (2-1) molecular
transition line (centered
at 230.538 GHz, upper sideband) on 2007 June 28. The phase
center of
the observations was
,
and
,
and the projected baselines ranged from 9 to 78 k
(12-101 m).
System temperatures ranged between 80 and 200 K. The
zenith opacities, measured with the NRAO tipping radiometer located
at the Caltech Submillimeter Observatory, were good during the track,
with
(225 GHz)
.
The correlator, with a bandwidth of
1.968 GHz, was set to the standard mode, which provided a
spectral
resolution of 0.8125 MHz (or 1.06 km s-1
per channel) across the
full bandwidth. The FWHM of the primary beam at 230 GHz was
.
The flagging and calibration of the data were done
with the MIR-IDL
package. The
passband response was obtained from observations of 3C454.3. The
baseline-based calibration of the amplitudes and phases was performed
using the sources 0102+584 and 0014+612. Flux calibration was set by
using Uranus, and the uncertainty in the absolute flux density scale
was
20%. Imaging
and data analysis were conducted using the
standard procedures in MIRIAD (Sault et al. 1995) and AIPS
(see Table 1
for details). The continuum was obtained by averaging all the
line-free channels of the upper sideband and the lower sideband.
3 Results
3.1 Centimeter continuum emission
We detected centimeter radio continuum emission at all wavelengths. In
Fig. 1a
we show the 6 cm continuum emission of the region.
The field is dominated by a strong and compact source associated with
the brightest infrared source in the field, which is the counterpart of
the UCH II region detected at
3.6 cm by Sánchez-Monge
et al. (2008).
Additionally, the wide field of the VLA at 6 cm allows us
to detect two unresolved sources to the north and east of the
UCH II region. The northern one, with the
coordinates of
(J2000.0) = 00
14
03
.21,
and
(J2000.0) = +64
32
26
.5,
has a primary beam
corrected flux density of
mJy. The eastern
one, with the coordinates of
(J2000.0) = 00
14
57
.69,
and
(J2000.0) = +64
28
49
.6,
has a primary beam
corrected flux density of
mJy.
Both sources are outside the
region shown in Fig. 1a.
At 3.6 cm, we improved the angular resolution of
previous observations
by a factor of five. These observations reveal two sources in the
field: an eastern source coincident with the UCH II
region, and a
second and fainter source located 15'' to the
west. Figure 1b
shows the resulting image after combining
the new VLA C-configuration observations with previous VLA
D-configuration observations (Sánchez-Monge
et al. 2008).
Since the combined dataset has a better
uv-coverage, we recovered faint structure to the
south of the
UCH II region similar to the double-source
structure detected at
1.3 cm by Sánchez-Monge et al. (2008). The
faint
source to the west of the UCH II region is
coincident with the main
1.2 mm peak detected with the IRAM 30 m
telescope (Sánchez-Monge
et al. 2008).
By combining the 3.6 cm VLA-C and VLA-D
datasets with archival VLA-B data we improved the angular resolution up
to
1.8''. With
this angular resolution, the faint source
15'' to the west of the UCH II region is
marginally detected, and
the UCH II region shows a shell morphology
with three main peaks and
with the 2MASS source falling at the center of the peaks (see
Fig. 1g).
![]() |
Figure 2:
Channel maps of the isolated N2H+ (1-0)
hyperfine |
Open with DEXTER |
![]() |
Figure 3:
a) Zero-order moment (integrated intensity)
for the hyperfine |
Open with DEXTER |
In Fig. 1c we show, for completeness' sake, the 1.3 cm continuum emission map from Sánchez-Monge et al. (2008). At 7 mm we detected one compact source at the position of the UCH II region (Fig. 1d).
In Table 2 we summarize the main results of the sources detected in the region at different wavelengths. The table gives for each source the coordinates, peak intensity, flux density, deconvolved size and position angle.
3.2 Millimeter continuum emission
Figure 1e
shows the 3.2 mm PdBI continuum emission of the
region. There are two compact strong sources at 3.2 mm: the
strongest
source, MM1, lying 15''
to the west of the UCH II region; and
the other source, MM2, being located
15'' to the south of MM1.
The peak of MM1 is associated with the faint 3.6 cm source
shown in
Fig. 1b
and with a near-infrared 2MASS source. MM1 is
partially extended to the southwest, also spatially coincident with a
second infrared source detected in the 2MASS (J00142558+6428416),
hereafter 2M0014256 (cf. Fig. 1e). Note that the
-2MASS image
shows no infrared emission toward
MM2 (cf. Fig. 1a).
The SMA map at 1.3 mm shows two compact sources
clearly associated
with MM1 and MM2 (Fig. 1f).
The 1.2 mm maps from the PdBI,
with an angular resolution three times better than the SMA images, but
only covering the region of MM1 within the primary beam of 22'',
show that this source splits up into at least three subcondensations:
a compact core, MM1-main, elongated in the east-west direction and
with faint extensions towards the north and the south; a faint
4
source about 1'' to the south of MM1-main, MM1-S; and a
faint source at 5
located
2''
to the southwest of
MM1-main, MM1-SW (see Fig. 1h
and
Table 3).
We note that the PdBI 1 mm and SMA 1 mm
flux densities are completely consistent, if we take into account that
the PdBI is filtering out emission at smaller scales than the
SMA. Throughout all the paper we will refer to MM1-main as MM1.
In Table 2
we summarize the main results of the sources
detected in the region at millimeter wavelengths (see Sect. 3.1
for a description of the table). For both MM1 and MM2, we estimated
the mass of the dust component assuming dust temperatures of
30 K and
20 K for MM1 and MM2, respectively (as a first approximation,
since
MM1 is associated with near-infrared emission, while MM2 is not), and
a dust mass opacity coefficient at 1.3 mm of
0.9 g cm-1(agglomerated grains
with thin ice mantles in cores of densities
106 cm-3,
Ossenkopf & Henning
1994).
With these assumptions, we estimated a
mass for MM1 of
3.0
,
and a mass for MM2 of
1.7
(both
derived from the (SMA) flux at 1.3 mm given in
Table 2).
In order to properly estimate the spectral
index between 3.2 mm (PdBI) and 1.3 mm (SMA), we made
the SMA image
using the same uv-range as the PdBI data
(9-72 k
).
By
measuring the flux densities in the maps in the common uv-range,
we
obtained a spectral index of
for MM1 and
for
MM2. Finally, we estimated the fraction of missing flux resolved out
by the SMA at 1.3 mm by comparing the total flux of MM1 plus
MM2 with
the flux density measured with the IRAM 30 m
Telescope of 1.2 Jy
(Sánchez-Monge et al. 2008), and
found that the
fraction of flux filtered out by the SMA is 92%.
3.3 N2H+ emission
Figure 2
shows the N2H+ (1-0)
channel maps of the
hyperfine
line (hereafter, the ``isolated'' hyperfine) toward
IRAS 00117+6412. Close to the systemic velocity
and spanning
2 km s-1,
the emission
appears as a ridge at the MM1 position (labeled as the MM1 ridge) and
is
elongated roughly in the east-west direction (PA
). While
at redshifted velocities the emission is only associated with MM2
(hereafter, the MM2 clump), the most blueshifted velocities show some
faint emission
20''
towards the west of MM2 (western
clump). The zero-order moment map integrated for the isolated hyperfine
of N2H+(1-0) is presented
in Fig. 3a,
overlaid on a
-band
image of 2MASS. The MM1 ridge has a
length of 20'' and the crest is coinciding well with the peak and
elongation of the 3.2 mm continuum emission (Fig. 1e). As
for the MM2 clump, it has a size of
10'', with the peak
position also coinciding with the millimeter peak, and is elongated in
the southeast-northwest direction (PA
). Finally, it is
worth noting that the MM2 clump and the western clump fall at a region
which is completely dark in the near-infrared, as seen in the
-2MASS image
of Fig. 3a.
3.4 CO (2-1) emission
Channel maps of the CO (2-1) emission are displayed in Fig. 4. The CO (2-1) emission appears spanning a wide range of velocities, from -70 up to -14 km s-1 (systemic velocity at -36.3 km s-1). Blueshifted emission appears in different clumps toward the north and northeast of MM1, while redshifted emission appears toward the southwest of MM1, suggesting a bipolar structure centered on MM1.
The spectrum of the emission integrated over all the region is
shown
in Fig. 5,
together with a preliminary spectrum
of 13CO (Busquet et al., in prep.).
The CO spectrum shows a dip from -6 up to +6 km s-1
with respect to the systemic velocity,
which could be due to self-absorption by cold foreground gas or
opacity effects, as the dip is coincident with the peak of the
13CO line. We note that the dip could be
partially produced by
the missing short-spacing information in the interferometer data as
well. In order to make a rough estimate of the fraction of flux
filtered out by the SMA (due to the lack of uv
sampling at spacings
smaller than 9 k),
we compared the single-dish spectrum (from
Zhang et al. 2005)
at certain velocities with the SMA
spectrum (extracted from the channel maps of
Fig. 4
convolved to the single-dish beam, of
30'') at the position of the IRAS source. The missing flux is
around 99% for the systemic velocities, indicating that at these
velocities the CO emission mainly comes from structures much larger
than
10'', which
is the largest angular scale observable by an
interferometer whose shortest baseline is
9 k
(see
Appendix for details). On the other hand, the missing flux
decreases as one moves to higher velocities. For example, the fraction
of missing flux goes down to 50% at -7 and 5 km s-1
(with respect to
the systemic velocity) for the blueshifted and redshifted sides,
respectively; and the SMA recovers all the single-dish flux at -13and
7 km s-1 with respect to the
systemic velocity. This indicates that
the high-velocity CO (2-1) emission has a characteristic
source size
10''.
4 Analysis
![]() |
Figure 4:
CO (2-1) channel maps of the IRAS 00117+6412 region,
averaged over
3.1 km s-1 wide velocity
intervals. The central velocity of each channel is
indicated in the upper left corner, and the systemic velocity is
-36.3 km s-1.
Symbols are the same as in Fig. 1. The synthesized
beam, shown
in the bottom left corner of each panel, is
|
Open with DEXTER |
4.1 N2H+ kinematics: moments and pv-plots
In order to study the kinematics of the dense gas as traced by N2H+, we computed the first-order moment for the isolated hyperfine of N2H+(1-0), which does not suffer from blending with other hyperfines (Fig. 3b). The first-order moment map shows a velocity gradient in the MM1 ridge, spanning velocities of -43.5 to -45.0 km s-1, centered approximately on MM1, and perpendicular to the elongation of the ridge. Another clear velocity gradient can be seen toward MM2 at red velocities, from -42 to -44 km s-1. Note that both velocity gradients observed toward MM1 and MM2 have the blueshifted velocities toward the northwest and the redshifted velocities toward the southeast. In Fig. 3c we show the second-order moment map of the region, tracing the velocity dispersion, which is largest toward MM1 and MM2, and in general along the MM1 ridge, reaching (linewidth) values of 0.9-1.4 km s-1, much larger than the thermal linewidth for N2H+, of around 0.22 km s-1 at 30 K. The western clump is the more quiescent emission in the region, with linewidths of up to 0.6 km s-1. In addition, the first-order moment reveals an abrupt change in velocity in the western clump (Fig. 3b), which could be indicative of the clump being kinematically decoupled from the MM1 ridge and the MM2 clump.
In Fig. 6
we show the N2H+ spectra
for MM1, for a
position in the northeastern (NE) side of the MM1 ridge (see positions
in Fig. 3b),
and for MM2. Note that the lines are broad,
as can be seen in the isolated hyperfine line (at -44 km s-1),
and suggestive of a double velocity component. See Sects. 4.2
and 4.3 for a
further analysis and discussion on these
spectra.
We additionally built position-velocity (pv) cuts to further
study the
kinematics of the region. In Fig. 7 we show two cuts
in
the direction of the velocity gradient seen in the MM1 ridge (at
PA
,
for MM1 and a position to the NE of MM1, see
positions in Fig. 3b),
and one cut along the elongation
of the ridge (PA = 40
). The three cuts show emission
spanning from
-43 to
-45 km s-1,
with one compact component at the blueshifted
velocities of (-44.2)-(-45.0) km s-1,
and a broad red component at
-43.5 km s-1.
The emission from the NE position and from MM1 follow the same general
trend of a blue component with a small velocity gradient and a red
broad component with no gradient (better seen in the cut of the NE
position, Fig. 7a).
For the perpendicular cut (PA = 40
,
centered on MM1, Fig. 7c),
we also find a velocity
gradient with the redshifted velocities at negative values, or to the
southwest of the cut, and the blueshifted toward the northeast.
Regarding MM2, the velocity gradient is also measured at a
PA = 130
,
coincident with the elongation of the MM2 clump
(Fig. 3b),
and is well seen in the pv-plot at
PA = 130
,
with a double peak
(Fig. 8-top).
However, the perpendicular cut does not
show any gradient, although the double peak is also there. We refer
the reader to the subsequent sections for the interpretation of the
kinematics of the N2H+
dense gas toward MM1 and MM2.
4.2 N2H+ kinematics: MM1 ridge
Table 4: Parameters of the hyperfine fits to the N2H+ (1-0) line.
In order to fit the hyperfine spectra toward MM1 and the NE position of the ridge (spectra shown in Fig. 6), we assumed two velocity components. We initially fitted two Gaussian to the isolated line to estimate the initial values of the velocities and linewidths for the two velocity components. With these initial values, we fitted the hyperfine spectra with two velocity components for the NE position and MM1. The results are listed in Table 4 and shown in Fig. 6. The fits can reproduce the data reasonably well.
For MM1, the derived two velocity components at
-36.7 km s-1, with a
linewidth of 0.7 km s-1,
and -35.9 km s-1, with a
linewidth of
1.4 km s-1,
were already seen in the pv-plots of MM1 at
PA = 130
shown in Fig. 7
(note that in
Fig. 7
we show the isolated line, which was not
taken as the reference line, and the component fitted at
-36.7 km s-1 corresponds to
-44.7 km s-1, while the
component at
-35.9 km s-1 corresponds to
-43.9 km s-1 in the pv-plot).
For the NE
position of the ridge, the two velocity components are found again
around -36.7 and -35.6 km s-1,
and the redshifted component has a
broader linewidth than the blueshifted component, as in the MM1
spectrum. We note that the red component is optically thinner than the
main component. This red component, which shows no velocity gradient,
could be tracing a filament intercepting the line of sight maybe
connecting the MM1 ridge and MM2, as the velocity of this red
component is similar to the velocity of MM2. Finally, we showed in
Sect. 4.1
(Fig. 7a)
that the blue component
(corresponding to the strongest component in the spectra) has a small
velocity gradient which could be tracing the global rotation of the
MM1 ridge.
![]() |
Figure 5: Spectrum of the 12CO (2-1) (thick line) and 13CO (2-1) (thin line) emission in the IRAS 00117+6412 region, averaging over all emission of outflow lobes. Blue (solid) and red (dashed) vertical lines indicate the range of velocities (red wing: -30.0 km s-1 up to -8.0 km s-1, and blue wing: -72.0 km s-1 up to -42.5 km s-1) used to estimate the parameters of the outflow. |
Open with DEXTER |
![]() |
Figure 6:
Top: N2H+ (1-0)
hyperfine spectra toward MM1.
Middle: idem toward the NE position of the
MM1 ridge (
4.0'',2.5''with respect to the
phase center; see Fig. 3b).
Bottom: idem toward MM2.
For MM1, and the NE position, we fitted the hyperfine structure with
two velocity components, and for MM2 we used one single velocity
component.
In the middle panel, the blue arrows indicate the position of the seven
hyperfines (with statistical weights from Womack
et al. 1992
and frequencies from Caselli
et al. 1995)
for the velocity component at
-36.7 km s-1 (Table 4). The different
hyperfine lines
correspond, from left (blue velocities) to right (red velocities), to
the quantum numbers: |
Open with DEXTER |
In Table 4
we also list the excitation temperature derived
from the fits and the N2H+
column density calculated assuming that
the filling factor is 1
and following Caselli
et al. (2002b).
We note that the excitation temperature of
3 K,
and the N2H+ column
densities are similar to the values
found in low-mass star-forming regions (e.g., Caselli
et al. 2002a;
Chen et al. 2007;
Kirk
et al. 2009),
and slightly smaller than the values found in
massive star-forming regions observed with interferometers (e.g., Palau
et al. 2007a;
Beuther & Henning
2009).
The small excitation temperature obtained
is suggestive of either low density and/or cold gas.
Note however that our values could be affected by the missing flux
problem caused by the lack of short uv-spacings in
the
interferometric data (for the PdB, the largest angular scale
detectable is
11'',
see Appendix, while in Palau
et al. (2007a)
and Beuther & Henning
(2009),
the largest angular scale was 20-30'',
which means that our PdB data are more affected by the missing flux
problem).
4.3 N2H+ kinematics: MM2
We fitted the N2H+ (1-0)
spectrum toward MM2 with a single velocity
component (Fig. 6-bottom).
As can be seen in the
figure, there is some excess of emission at blueshifted velocities,
which could be contamination from the MM1 ridge (which has an
extension toward the south at blueshifted velocitites with respect to
the MM2 clump, at around (-44.6)-(-44.7) km s-1,
as seen in
Fig. 2).
Alghough the excess of emission is quite clear, a
fit with two velocity components for the MM2 N2H+
hyperfine spectrum
could not be well determined.
From the parameters obtained from the fit, we
derived the excitation temperature and N2H+
column density as in the
previous section, and the values found for MM2 are very similar to the
values found for MM1.
![]() |
Figure 7:
Position-velocity (pv) plots of the N2H+
isolated line between -46 and -42 km s-1,
toward positions of the MM1 ridge. a) Pv-plot in a
cut with PA = 130 |
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![]() |
Figure 8:
Position-velocity (pv) plot of the N2H+
isolated line toward MM2 in a cut of
PA = 130 |
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In order to further constrain the N2H+ kinematics toward MM2, we approached the flattened structure seen in N2H+ to a model of a spatially infinitely thin disk seen edge-on, with a power-law intensity as function of radius, consisting of a superposition of optically thin rings undergoing infall and rotation (also described as power-laws). We computed the synthetic pv-plots along the projected major and minor axes of the disk, with angular and spectral resolutions of 3.4'' and 0.35 km s-1, respectively. The modeled emission is shown in Fig. 8-bottom, and the adopted/fitted parameters are listed in Table 5.
The disk structure is modeled with an inner and outer radius. The outer radius is constrained from the N2H+ emission in the observed pv-plot, and is also taken as the reference radius for the rotation and infall power-laws. The intensity power-law index was adopted to be -1, as used in other models of disk-like structures around intermediate/high-mass YSOs (e.g., Beltrán et al. 2004). For the infall we assumed a free-fall infall velocity, and for the rotation law we assumed a solid rigid rotation, as recent studies show that these initial conditions are able to account for most observational properties of star-forming cores (e.g., Tscharnuter et al. 2009; Walch et al. 2009; Zhilkin et al. 2009). The free parameters are the reference infall velocity and the reference rotation velocity.
We constrained the upper limit of the inner radius from the
SMA map at
1 mm, which has an angular resolution of 2.5'', and
hence any
inner gap must have a radius
1.2''. On the other hand, the
N2H+ data further
constrain the inner radius to 0.8'', as a smaller
inner radius clearly cannot reproduce the double peak in the
PA = 130
pv-plot. We conclude that the inner radius covers the
range of 0.8-1.2'', and adopted the mean value of
.
We
derived the reference infall velocity from the PA = 40
pv-plot
(only affected by infall), of
km s-1.
We note that to
include infall in the model is necessary in order to reproduce the
double peak seen in the pv-plot at PA = 40
.
Finally, we fitted the
rotation velocity from the PA = 130
pv-plot (affected by both
infall and rotation). We note that in the PA = 130
pv-plot the
velocity component at
-43.5
could be contaminated by the MM1
ridge, and hence we focused our fit on the two central peaks at around
-43.0 km s-1. The resulting
reference rotation velocity is
km s-1.
We also note that if we fit the data adopting an
inner radius of 0.8'' or 1.2'', the new parameters are similar,
within the uncertainties, to the parameters derived for an inner
radius of 1''.
From the fitted infall velocity, one can estimate the mass of
the
central object by using
,
with ibeing the inclination angle to the plane of
the sky. Thus, taking
km s-1
at a reference radius of
,
and assuming an inclination of 90
,
we obtain a mass
,
where the lower limit accounts for the inclination
assumption. We note that the true mass however is likely not larger
than a few tenths of solar mass, because we estimated the inclination
of the MM2 N2H+ core from
its deconvolved major and minor axis
(
), and obtained an
inclination, assuming it is
intrinsically circular, of
80
,
which is very close to the
assumed inclination.
It is interesting to compare the mass derived from infall, which is
indicative of the mass already accreted onto the protostellar core,
with the mass derived from the millimeter continuum emission for MM2
(around 1.7
,
Sect. 3.2),
which is tracing the mass of the
envelope and disk surrounding the central core. For Class 0 sources
it is typically assumed that these two masses should be of the same
order. Thus, the obtained accreted mass smaller than the disk+envelope
mass is suggesting that the central protostellar object in MM2 has
just recently formed.
Table 5: Output parameters of the position-velocity plot modeled emission for MM2.
4.4 CO kinematics: moments and pv-plots
We constructed the zero-order moment maps of the CO emission
integrated over three different velocity ranges: (1) systemic
velocities from -6 to +6 km s-1
with respect to the systemic
velocity (Fig. 9-top);
(2) moderate velocities from
-6 to -15 km s-1, and from +6
to +15 km s-1 with respect to
the
systemic velocity (Fig. 9-middle); and
(3) high
velocities from -15 km s-1 to
most negative velocities, and from
+15 km s-1 to most positive
velocities with respect to the systemic
velocity (Fig. 9-bottom).
At systemic velocities the
emission appears concentrated in two main structures, a clump
associated with the N2H+
MM1 ridge, and a multipeaked-arch located to
the north of MM1, just bordering the N2H+
emission. At moderate and
high velocities, which are less affected by the missing short spacings
in the interferometric data, the emission has a bipolar structure
centered near the position of MM1 and is elongated in the
northeast-southwest direction (see Fig. 9-middle and
bottom). At blue moderate velocities, the emission splits up into two
main peaks: Ba, close to the position of MM1 and elongated in the
northeast-southwest direction, and Bb, located 10'' northwards
of MM1 and with a round shape. A third fainter clump, Bc, is located
10'' to the
northeast of MM1. Regarding the redshifted
emission, it appears as an elongated cone-like structure, with one
main clump, Ra, which is well aligned with the Ba clump, with a
position angle of
45
,
and centered
7''
toward the
southwest of MM1.
At higher velocities, the emission is found toward Ba, Bb, Ra, and
toward a second redshifted clump, Rb, which was not very prominent at
moderate velocities. Clumps Bb and Rb are aligned at a position angle
of
20
,
and centered near MM1. Note that the position
angles of the high-velocity CO emission are almost perpendicular to
the direction of the N2H+
velocity gradient, found at
130
.
![]() |
Figure 9:
Top: contours: CO (2-1) moment-zero
map integrated for
systemic velocities (between -6 and +6 km s-1
with respect to the
systemic velocity, -36.3 km s-1).
Levels start at 24%, increasing in
steps of 15% of the peak intensity, 22.9
|
Open with DEXTER |
![]() |
Figure 10:
Top: CO (2-1) pv-plot in the
northeast-southwest direction
with PA = 45 |
Open with DEXTER |
Table 6: Physical parameters of the outflow driven by MM1.
Table 7: 2MASS sources associated with the centimeter and/or millimeter emission toward the star-forming region IRAS 00117+6412.
![]() |
Figure 11: Spectral energy distribution in the centimeter and millimeter range for the UCH II region. Dashed line: free-free optically thin fit with a spectral index of -0.03. VLA, SMA and PdBI data from this work. IRAM 30 m data from Sánchez-Monge et al. (2008). |
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We performed different pv-plots close to MM1 at
PA = 45,
and
PA = 20
(see
Fig. 10),
in the two directions
indicated in Fig. 9.
Both pv-plots have been convolved
with a Gaussian (
at PA = 135
and 110
,
respectively). In both plots, positive and negative positions
correspond respectively to northeastern and southwestern positions
with respect to MM1. In the first cut (45
)
we can distinguish a
strong velocity gradient from clump Ra to clump Ba, with velocities
increasing with distance, following a Hubble-law pattern (up to
34 km s-1 with respect to the
systemic velocity in the case of clump
Ba). We note that in the cut at PA = 45
,
the clump Bc shows
moderate velocity emission of up to 14 km s-1
with respect to the
systemic velocity. Regarding the pv plot at PA = 20
,
the redshifted emission and the blueshifted clump Ba also follow a
Hubble-law, as in the cut at 45
.
We calculated the energetics of the outflow for each blue and
red
lobe separately (for the blue lobe we included both clumps Ba and
Bb), assuming that all the emission comes from a single outflow, and
listed the values in Table 6. The expressions
used to
calculate the outflow CO column density,
,
from the
transition
,
and the outflow mass,
,
are
given in Palau et al. (2007b).
We adopted an opacity in the
line wings of
2
(from preliminary 13CO data), and an
excitation temperature of
7 K,
estimated from the spectrum in
Fig. 5
(assuming that CO is optically thick and
adopting a line temperature of 2.7 K). For the red lobe we
integrated
from -30 to -8 km s-1, and for
the blue lobe from -72 to
-42.5 km s-1. The dynamical
timescale of the outflow,
,
was derived by dividing the size of each lobe by the
maximum velocity reached in the outflow with respect to the systemic
velocity (28.3 km s-1 for the
red lobe, and 35.7 km s-1 for
the blue
lobe).
4.5 IRAS, MSX and 2MASS emission
With the aim of finding the possible infrared counterparts of the
sources studied in this work, we searched the IRAS, MSX and 2MASS
surveys (the region has not been observed by Spitzer). While the IRAS
position error ellipse makes it difficult to disentangle the
contribution from the UCH II region and
MM1, we superposed the MSX
emission on our millimeter maps and found that the MSX emission is
compact and clearly peaking at the UCH II
region. Thus, this is
suggestive of the major part of the IRAS and MSX fluxes, at least up
to m, coming
from the UCH II region. However, at 60 and
100
m
the contribution of the UCH II region and
MM1 is not
clear, and for this reason we refrained from building the spectral
energy distributions. As for the 2MASS Point Source Catalog (Skrutskie
et al. 2006),
in Table 7
we show the 2MASS
photometry for the 2MASS counterparts of the UCH II
region, MM1, and
for 2M0014256. From the 2MASS magnitudes, we estimated the (J-H)
and
(
)
colors and measured the infrared excess as the
difference between the (
)
color and the
(
)
color corresponding to a reddened main-sequence
star (following the reddening law of Rieke & Lebofsky
1985).
The 2MASS sources associated with the
UCH II region and MM1 show a moderate
infrared excess typical of
Class II sources, while 2M0014256 seems to be a reddened
main-sequence
star, or a Class III source. A detailed analysis of the low-mass
content of the forming cluster in IRAS 00117+6412 will be presented in
a subsequent paper (Busquet et al., in prep.).
5 Discussion
5.1 A shell-like UCH II region
The centimeter range (from 6 cm up to 7 mm) of the
spectral energy
distribution of the UCH II region can be
fitted assuming free-free
optically thin emission from ionized gas with a spectral index of
(see Fig. 11;
the spectral index was
calculated from the fluxes measured in the images at 6, 3.6,
1.3 cm
and 7 mm made with the uv-range shared at
all the wavelengths,
4.35-55 k
).
We calculated the physical parameters of the
H II region at 3.6 cm (in the CD
configuration) assuming the emission
is optically thin. The results are consistent with a UCH II
region
with a size of
pc,
a brightness temperature of 12.4 K,
an emission measure of
cm-6 pc,
an electron density
of 2000 cm-3, a mass of inonized gas of
(estimated from the beam
averaged electron
density and the observed size of the source), and a flux of ionizing
photons of
s-1.
These parameteres are
consistent with a UCH II region driven by
an early-type B2 star
(from Panagia 1973),
as already stated by Sánchez-Monge et al. (2008).
The high angular resolution centimeter observations presented
in this
work reveal that the UCH II region reported
in Sánchez-Monge
et al. (2008)
already has a shell-like structure. The
average radius of the shell (from an average of the three
subcondensations) is
,
which corresponds to 2200 AU or
0.01 pc. From this radius, one can make a rough estimate of
the
dynamical timescale, either assuming a classical expansion of an HII
region at
10 km s-1,
or the expansion of a wind-driven bubble (as
typically assumed to explain the shell-like morphologies of
UCH II regions, e.g., Garay &
Lizano 1999).
In the
first case (classical HII region), the dynamical timescale is
1000 yr,
while in the second case (stellar wind blown bubble)
the dynamical timescale is
12000 yr
(assuming the stellar wind
dominates over the classical expansion of the UCH II
region, an
initial ambient density of
107 cm-3,
and following Castor
et al. 1975,
and Garay & Lizano 1999,
yielding an expanding velocity
1 km s-1).
The lifetimes
derived from both assumptions are much smaller than
1 Myr,
the
lifetime estimate for UCH II regions of
1000
,
from the RMS
survey by Mottram et al. 2009 (in prep.). This could be
indicative of
the UCH II region in
IRAS 00117+6412 having undergone in the past a
period of strong quenching (due to a high-mass accretion rate,
Walmsley et al. 1995),
before expanding as a classical
HII region or as a wind-blown bubble. Alternatively, if the classical
HII region expansion or the wind-blown bubble assumptions are correct,
we could be witnessing the very first stages of the expansion of an
ionized shell around a B2-type (
1000
)
YSO. However, the fact
that we do not detect significant dust or dense gas emission
associated with the UCH II region suggests
that the object is in the
process of disrupting its natal cloud, and most likely has a lifetime
similar to the
1 Myr
given by Mottram et al. (2009, in
prep.). In addition, diffuse optical emission can be seen to the
northeast of the UCH II region in the blue
and red plates of the
Palomar Observatory Sky Survey II, supporting the fact that the
UCH II region is not deeply embedded and is
emerging from its natal
cloud.
From the bolometric luminosity and the spectral type of the
UCH II region one can estimate the mass and
the evolution time from
the birthline by placing the object in a HR diagram. Following the
models of Palla & Stahler (1990,
1993),
and adopting an effective temperature of
20 500 K
(Panagia 1973),
the mass of the star
ionizing the UCH II region is about
5-6
,
and the star is already
placed at the ZAMS, which most likely is reached after
1 Myr
for
a star of this mass (e.g., Palla & Stahler 1993;
Bernasconi & Maeder 1996).
This estimate agrees with the previous estimate of the lifetime of a
UCH II region of
1000
(derived from Mottram
et al. 2009, in
prep.) of
1 Myr.
5.2 MM1: multiple sources and a spectacular outflow
In previous sections we showed that MM1 is associated with a 2MASS
source with infrared excess. Dense gas emission traced by N2H+
and
faint centimeter emission are also associated with MM1. These
properties suggest that MM1 is most likely in a phase of
moderate/intense accretion, possibly in the Class 0/I stage, with
typical ages of (0.5-1)
yr (e.g., Evans
et al. 2009).
On the other hand, the estimated envelope mass of
3.0
and the
magnitude of the near-infrared emission (similar to the magnitudes of
the UCH II region, which has a luminosity
of
1000
)
are
suggestive of MM1 being possibly an intermediate-mass YSO.
In particular, from the outflow momentum rate (
)
we can
estimate the luminosity of the object powering the outflow (assuming
one single outflow). Using the relation from Hatchell
et al. (2007)
we obtain
,
and
with the relation from Bontemps et al. (1996) we
obtain
,
consistent with a driving source of
intermediate mass.
We note that concerning the driving source(s) of the outflow(s), we discard 2M0014256 as a possible candidate because its near-infrared excess is much smaller than that of MM1 (see Table 7), and lies slightly off from the line joining the high-velocity clumps Ba and Ra, or Bb and Rb. The only aspect suggestive of 2M0014256 being associated with the star-forming region is the elongation seen at 3.2 mm, which matches well with the position of the 2MASS source (see Fig. 1e). However, such an elongation at 3.2 mm could be tracing dust entrained by the outflow, as the elongation and the Ba and Ra lobes are very well aligned. If 2M0014256 was really associated with the region one would expect some compact emission associated with it in the 1 mm continuum maps (which have higher angular resolution than the 3.2 mm continuum maps), but as this is not found, it suggests that the elongation is made of extended emission.
Another aspect favoring the scenario that MM1 could be the
driving
source of the outflow is that MM1 is associated with centimeter
emission, typically found toward Class 0/I sources driving
outflows. Since we only detect the source in the centimeter range at
3.6 cm we can only estimate a range of possible spectral
indices,
,
which is consistent with thermal
free-free emission from ionized gas (the spectral index was calculated
from the images at 6, 3.6, and 1.3 cm made with the same
uv-range). The fact that the centimeter source is
elongated in a
position angle of
,
almost perpendicular to the CO
outflow, could be due to contribution from the different
subcondensations associated with MM1. Additionally, from the work of
Shirley et al. (2007),
we estimated the predicted
centimeter flux which can be accounted for from the outflow momentum
rate, and obtained a flux density at 3.6 cm of
0.3 mJy.
Since we
measured a flux density at 3.6 cm of 0.17 mJy, we
conclude that all
the centimeter emission associated with MM1 can be accounted for
through ionization by shocks from the outflow.
The interpretation of the high-velocity CO bipolar outflow and
the
identification of the driving source(s) are not straightforward. A
possibility is that clumps Ba and Ra are tracing an outflow with a
position angle of 45
powered by one of the subcondensations of
MM1, maybe MM1-S, as it falls exactly at the center of symmetry of
clumps Ba and Ra (although it is less massive than MM1-main); and that
Bb and Rb are tracing a second outflow with a position angle of
20
,
possibly powered by MM1-main, as it shows an elongated
structure in the east-west direction, almost perpendicular to the
Bb-Rb outflow. Another possibility is that all the CO emission comes
from a single wide-angle outflow, which would be driven by MM1-main
(since it shows an east-west elongation), and would be excavating a
cavity, seen at moderate velocities. If this was true, the eastern
wall of the cavity, corresponding to clump Ba, would have very
high-velocity emission (up to
35 km s-1
with respect to the
systemic velocity). A similar case in the literature is found for
G240.31+0.07 (Qiu et al. 2009),
a massive YSO driving a
wide-angle outflow, and with velocity in the walls of up to
27 km s-1 with respect to the
systemic velocity. Such a high velocity
in the cavity wall could be explained through precession of the
outflow axis and episodic mass ejection. Arce & Goodman
(2001)
study the position-velocity relation for
episodic outflows and show that clumps at different positions from the
driving source are expected. Each one of these clumps shows a wide
range of velocities (see e.g., Fig. 2 in Arce &
Goodman
2001).
This can also be seen in the pv-plot of
IRAS 00117+6412 at 20
(Fig. 10-bottom).
In the
figure, both the blueshifted and the redshifted sides show a pair of
clumps, each one spanning a range of
20 km s-1.
Thus, both
possibilities seem plausible for this target, both mutiple outflows
driven by different subcondensations of MM1, and a single wide-angle
episodic outflow.
5.3 MM2: an intriguing protostellar object
In Sect. 4.3
we modeled the N2H+
emission toward MM2 as a
disk structure undergoing infall and rotation. The infall and rotation
velocities (at 5'') can be compared with those derived from other
protostellar cores. Beltrán et al. (2005) model,
for the
high-mass case for example, two massive cores with infall velocities
of 1-2 km s-1, and rotation
velocities of km s-1,
at spatial
scales of 5000-10 000 AU. On the other hand, infall
and rotation
velocities for low/intermediate-mass protostars are around 0.1-0.4
and 0.2 km s-1, respectively,
at spatial scales of
3000 AU
(Beltrán et al. 2004;
Ward-Thompson
et al. 2007;
Carolan et al. 2008).
From
the infall and rotation velocities at the reference radius derived in
Sect. 4.3
for MM2, and using the assumed infall and rotation
power laws (Table 5),
we found an infall and rotation
velocity of 0.29 and 0.2 km s-1
respectively at 3000 AU, which is
more similar to the velocities derived for low/intermediate-mass cores
than to the velocities derived for high-mass cores.
In addition, the flattened structure was modeled with an inner gap of
1''. We note
that this inner gap must not necessarily be a
physically real gap. First, it could be an opacity effect. However,
this possiblity can be discarded because the opacity of the
isolated line (the hyperfine used to fit the model in
Sect. 4.3)
estimated from the fits is
0.05
(see
Table 4),
which is clearly optically thin.
Second, a more plausible option is that the N2H+
gap is reflecting
the depletion of N2H+ in
the center, maybe due to freezing-out of
N2H+ due to high
densities and low temperatures in the center, as
found in very young (sometimes pre-protostellar) and dense
(
cm-3)
regions (Belloche & André
2004;
Pagani et al. 2007).
High angular
resolution observations of the continuum millimeter emission would
help assess the nature of this N2H+
gap.
It is worth noting that the properties of MM2 are quite different from those of MM1. MM2 has no infrared emission and is embedded in a dusty compact condensation and in a dense gas clump, which seems to be undergoing rotation and infall motions. As there is water maser emission associated with the MM2 peak (see Fig. 1f), indicative of stellar activity, one would classify this source as Class 0, judging from its lack of infrared emission and its strong and compact millimeter continuum emission. However, YSOs in the Class 0 evolutionary stage are typically associated with strong and collimated molecular outflows (e.g., Bachiller 1996), while we found no hints of (high-velocity) CO associated with MM2. Furthermore, water masers associated with YSOs are related to accretion or ejection processes (e.g., Furuya et al. 2003), and observational studies suggest a connection between water masers and the outflow phenomenon (e.g., Zhang et al. 2001). Thus, the lack of outflow emission associated with MM2 is intriguing. Different possibilities could explain this behavior (lack of outflow in an apparent Class 0 source with water maser emission). First, the object could be driving an outflow very faint in CO, but which could be well traced by other molecules such as SiO (studies of high-mass star-forming regions show that some molecular outflows are faint or not detected in CO while they are strong in SiO, and viceversa, Beuther et al. 2004; Zapata et al. 2006). However, a preliminary reduction of SiO (1-0) data observed with the VLA (Busquet et al., in prep.) show no outflow hints. A second possibility would be that the outflow has not been created yet (the protostellar wind would still be in the phase of sweeping the ambient material out). The caveat with this possibility is that observations seem to indicate that outflows appear at the very first stages of the protostar formation (e.g., Bachiller 1996), and the probability of witnessing such a short-lived phenomenon is low. Third, the ejection process in this source could be different from the standard paradigm of a strong and collimated outflow associated with a Class 0 source. This is found for some objects in massive star-forming regions (Torrelles et al. 2001, 2003), which show spherical ejections traced by water masers. Thus, the true evolutionary stage of MM2 remains an open question. A study of the water maser emission with the highest angular resolution available would be of great help to elucidate the process of accretion/ejection in this enigmatic object.
In order to make a rough estimate of the MM2 luminosity we
used the
water maser luminosity reported by Wouterloot
et al. (1993)
toward MM2, of
,
which we corrected to our adopted distance. From a correlation between
the water maser luminosity and the bolometric luminosity of the YSO
associated with the maser (Furuya et al. 2003,
2007), we
estimated a luminosity for MM2 of
600
,
suggesting that MM2 is of intermediate-mass.
5.4 Different objects emerging from the same natal cloud
The results obtained toward IRAS 00117+6412 reveal that the
dusty
cloud harbours about three intermediate-mass YSOs showing different
properties. One of the sources seems to be an intermediate-mass
UCH II region with a shell-like structure,
located at the eastern
border of the dusty cloud, and almost deprived from dust and dense
gas, with an estimated luminosity of 1000
,
and an estimated
age of
1 Myr.
Another source, MM1, is deeply embedded within the
dusty cloud, has about 400-600
,
and an estimated age around
105 yr.
Thus, MM1 presumably formed after the
UCH II region. Finally, MM2, of about
600
,
remains the most
enigmatic object in this region, as it seems to be deeply embedded in
gas
and dust and has a water maser associated, but no signs of CO
outflow activity. This could be
a new type of object undergoing a special process of matter ejection
(such as spherical mass ejection).
In summary, our observations show that the formation of stars within the nascent cluster in IRAS 00117+6412 seems to take place in different episodes. In addition, these observations show that the similar initial conditions within a cloud (similar dust mass, excitation temperature, and dense gas column density, as found for MM1 and MM2) can yield objects with very different properties. This indicates that these initial conditions may not be decisive in determining some of the properties of the YSOs forming within the cloud, such as the ejection properties, a result already found at much smaller spatial scales by Torrelles et al. (2001, 2003).
6 Conclusions
In this paper we study with high angular resolution the centimeter, and millimeter continuum, and N2H+ (1-0), and CO (2-1) emission of the intermediate-mass YSOs forming within a dusty cloud, with the goal of assessing the role of the initial conditions in the star formation process in clusters. Our conclusions can be summarized as follows:
- 1.
- A UCH II region is found at the
eastern border of the dusty cloud, with a shell-like structure and a
flat spectral index,
. The estimated age and mass of the underlying star is
1 Myr and
6
.
- 2.
- Deeply embedded within the dusty cloud, we have discovered
a millimeter source, MM1, associated with a 2MASS infrared source,
which is driving a CO (2-1) powerful and collimated
high-velocity outflow, oriented in the southwest-northeast direction.
The mass derived from the millimeter continuum emission for MM1 is
3
. MM1 is embedded within a ridge of dense gas as traced by N2H+, which seems to be rotating roughly along the outflow axis. MM1 is associated with centimeter emission, whose spectral index is compatible with an ionized wind, and at 1.2 mm splits up into different subcomponents when observed with an angular resolution of
1''. From the derived outflow momentum rate, we estimated a luminosity for MM1 of 400-600
. Thus, MM1 seems to be a Class 0/I intermediate-mass YSO.
- 3.
- About
15'' to the south of MM1, our observations revealed a dust compact condensation, MM2, lying in a dark infrared region, associated with water maser emission and a dense core traced by N2H+ emission. The mass from the dust emission is
1.7
, and the N2H+ excitation temperature and column density are similar to the ones derived for MM1. The dense core in MM2 is rotating in the same sense as the ridge associated with MM1 and seems to be undergoing infall. We modeled the MM2 dense core as a disk-like structure with an inner radius of
1'' and an outer radius of
5'', with a rotation velocity in the outer radius of
km s-1, and an infall velocity at the same radius of
km s-1. The non-detection of CO at any velocity toward MM2 makes this object intriguing.
- 4.
- Although MM1 and MM2 formed within the same cloud and have similar dust and dense gas emission, their properties, specially concerning the ejection phenomenon, seem to be different and could be indicating that the initial conditions in a cloud forming a cluster are not the only agent determining the properties of the members of the cluster.
A.P. is grateful to Itziar de Gregorio-Monsalvo for useful discussions. A.P. is partially supported by the MICINN grant ESP2007-65475-C02-02, the program ASTRID S0505/ESP-0361 from La Comunidad de Madrid and the Europan Social Fund, and the Spanish MICINN under the Consolider-Ingenio 2010 Program grant CSD2006-00070. A.P., A.S.-M., G.B. and R.E. are supported by the Spanish MICINN grant AYA2005-08523-C03, and the MICINN grant AYA2008-06189-C03 (co-funded with FEDER funds). This publication 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.
Appendix A: Emission filtered out by an interferometer
In this appendix we describe the estimation of the fraction of flux
filtered out by an interferometer. Consider a bidimensional Gaussian
source with the flux density
and the half-power diameter D. Its
intensity, I(x,y),
can be expressed as
The visibility, V(u,v), of the Gaussian source corresponds to the fourier transform of the intensity (Eq. (A.1)),
and thus, the half-power (u,v) radius, r1/2, is
or in practical units, r1/2 in


From Eq. (A.4) we can estimate the largest structure,


From Eqs. (A.2) and (A.5) we can estimate the fraction of correlated flux,


with



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Footnotes
- ... IRAS 00117+6412
- The fits files for Figs. 1, 2, and 4 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/510/A5
- ...
(VLA
- The Very Large Array (VLA) is operated by the National Radio Astronomy Observatory (NRAO), a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
- ... (PdBI
- The Plateau de Bure Interferometer (PdBI) is operated by the Institut de Radioastronomie Millimetrique (IRAM), which is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
- ... (SMA
- The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and the Academia Sinica.
- ... MIR-IDL
- The MIR cookbook by Charlie Qi can be found at http://cfa-www.harvard.edu/ cqi/mircook.html
- ... velocity
- The systemic velocity for IRAS 00117+6412 is
-36.3 km s-1 (Molinari
et al. 1996;
Zhang et al. 2005),
and the N2H+ (1-0)
hyperfine adopted as reference line in this work was
. However, since the hyperfine adopted as reference is blended with two other hyperfines, we will make our analysis with the hyperfine which is isolated, the
line, which is at 8 km s-1 with respect to the line adopted as reference. Therefore, the reference velocity (corresponding to the systemic velocity) in the analysis of the N2H+ emission from the isolated hyperfine is -36.3-8.0=-44.3 km s-1.
- ... determined
- This is because the two-velocity-components hyperfine fit is strongly dependent on the adopted initial values. For the case of MM1 and the NE position of the MM1 ridge, this problem was solved by fitting two Gaussian to the line which is isolated, for which both velocity components are well detected. However, in the case of MM2, the velocity component at -44.5 km s-1 is barely detected in the isolated line (see Fig. 6-bottom), hindering the estimate of the initial values.
All Tables
Table 1: Main parameters of the VLA, PdBI, and SMA observations.
Table 2: Multiwavelength results for the intermediate-mass YSOs in the star-forming region IRAS 00117+6412.
Table 3: Parameters of the 1.2 mm subcondensations associated with MM1 from the uniform-weighted PdBI mapa.
Table 4: Parameters of the hyperfine fits to the N2H+ (1-0) line.
Table 5: Output parameters of the position-velocity plot modeled emission for MM2.
Table 6: Physical parameters of the outflow driven by MM1.
Table 7: 2MASS sources associated with the centimeter and/or millimeter emission toward the star-forming region IRAS 00117+6412.
All Figures
![]() |
Figure 1:
IRAS 00117+6412 continuum maps.
a) Grey-scale: 2MASS |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Channel maps of the isolated N2H+ (1-0)
hyperfine |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
a) Zero-order moment (integrated intensity)
for the hyperfine |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
CO (2-1) channel maps of the IRAS 00117+6412 region,
averaged over
3.1 km s-1 wide velocity
intervals. The central velocity of each channel is
indicated in the upper left corner, and the systemic velocity is
-36.3 km s-1.
Symbols are the same as in Fig. 1. The synthesized
beam, shown
in the bottom left corner of each panel, is
|
Open with DEXTER | |
In the text |
![]() |
Figure 5: Spectrum of the 12CO (2-1) (thick line) and 13CO (2-1) (thin line) emission in the IRAS 00117+6412 region, averaging over all emission of outflow lobes. Blue (solid) and red (dashed) vertical lines indicate the range of velocities (red wing: -30.0 km s-1 up to -8.0 km s-1, and blue wing: -72.0 km s-1 up to -42.5 km s-1) used to estimate the parameters of the outflow. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Top: N2H+ (1-0)
hyperfine spectra toward MM1.
Middle: idem toward the NE position of the
MM1 ridge (
4.0'',2.5''with respect to the
phase center; see Fig. 3b).
Bottom: idem toward MM2.
For MM1, and the NE position, we fitted the hyperfine structure with
two velocity components, and for MM2 we used one single velocity
component.
In the middle panel, the blue arrows indicate the position of the seven
hyperfines (with statistical weights from Womack
et al. 1992
and frequencies from Caselli
et al. 1995)
for the velocity component at
-36.7 km s-1 (Table 4). The different
hyperfine lines
correspond, from left (blue velocities) to right (red velocities), to
the quantum numbers: |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Position-velocity (pv) plots of the N2H+
isolated line between -46 and -42 km s-1,
toward positions of the MM1 ridge. a) Pv-plot in a
cut with PA = 130 |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Position-velocity (pv) plot of the N2H+
isolated line toward MM2 in a cut of
PA = 130 |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Top: contours: CO (2-1) moment-zero
map integrated for
systemic velocities (between -6 and +6 km s-1
with respect to the
systemic velocity, -36.3 km s-1).
Levels start at 24%, increasing in
steps of 15% of the peak intensity, 22.9
|
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Top: CO (2-1) pv-plot in the
northeast-southwest direction
with PA = 45 |
Open with DEXTER | |
In the text |
![]() |
Figure 11: Spectral energy distribution in the centimeter and millimeter range for the UCH II region. Dashed line: free-free optically thin fit with a spectral index of -0.03. VLA, SMA and PdBI data from this work. IRAM 30 m data from Sánchez-Monge et al. (2008). |
Open with DEXTER | |
In the text |
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