A&A 413, 609-622 (2004)
DOI: 10.1051/0004-6361:20031572
V. Wakelam1 - A. Castets1 - C. Ceccarelli1,2 - B. Lefloch2 - E. Caux3 - L. Pagani4
1 - Observatoire de Bordeaux, BP 89, 33270 Floirac, France
2 -
Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53,
38041 Grenoble Cedex 9, France
3 -
CESR CNRS-UPS, BP 4346, 31028, Toulouse Cedex 04, France
4 -
LERMA & FRE 2460 du CNRS, Observatoire de Paris, 61 Av. de
l'Observatoire, 75014 Paris, France
Received 10 June 2002 / Accepted 7 October 2003
Abstract
We report observations of the expected main S-bearing
species (SO, SO2 and H2S) in the low-mass star forming region
L1689N. We obtained large scale (
)
maps of
several transitions from these molecules with the goal to study the
sulphur chemistry, i.e. how the relative abundances change in the
different physical conditions found in L1689N. We identified eight
interesting regions, where we carried out a quantitative comparative
study: the molecular cloud (as reference position), five shocked
regions caused by the interaction of the molecular outflows with the
cloud, and the two protostars IRAS 16293-2422 and 16293E. In the
cloud we carefully computed the gas temperature and density by means
of a non-LTE LVG code, while in other regions we used previous
results. We hence derived the column density of SO, SO2 and H2S, together with SiO and H2CO - which were observed previously
- and their relevant abundance ratios. We find that SiO is the
molecule that shows the largest abundance variations in the shocked
regions, whereas S-bearing molecules show more moderate variations.
Remarkably, the region of the brightest SiO emission in L1689N
is undetected in SO2, H2S and H2CO and only marginally detected in SO. In the other weaker SiO
shocks, SO2 is enhanced with respect to SO. We propose a schema in
which the different molecular ratios correspond to different ages of
the shocks. Finally, we find that SO, SO2 and H2S have
significant abundance jumps in the inner hot core of IRAS 16293-2422
and discuss the implications of the measured abundances.
Key words: ISM: abundances - ISM: molecules - stars: formation - ISM: individual objects: L1689N - ISM: individual objects: IRAS 16293-2422
Low mass star forming regions are composed by at least three main ingredients: the molecular cloud from which protostars are born, the protostars themselves, and the shocked regions at the interface between the cloud and the outflows emanating from the protostars. These three regions have very different physical conditions, where temperature, density and also chemical abundances greatly differ (e.g. van Dishoeck & Blake 1998). This paper focuses on the abundance changes occurring to the S-bearing molecules and the relevant sulphur chemistry. Depending on the physical condition of the gas, it is believed that different types of reactions play a role in the formation of sulphur-bearing molecules. In molecular clouds, ion-molecule reactions are the most important (Prasad & Huntress 1982; Millar & Herbst 1990; Oppenheimer & Dalgarno 1974), whereas in the warm gas of the hot cores and shocks, neutral-neutral reactions play the major role in forming sulphur species (Charnley 1997; Keane et al. 2001; Hatchell et al. 1998; Pineau Des Forêts et al. 1993). Specifically, in warm gas, the abundances of H2S, SO and SO2 are supposed to increase significantly. This is the reason why they are often used to trace shocks (Chernin et al. 1994; Bachiller & Perez Gutierrez 1997; Pineau Des Forêts et al. 1993). And because of the relatively fast evolution of their chemistry, on time scale of tens of thousand years, they are good candidates to be chemical clocks to study the evolution of outflows (Bachiller et al. 2001) and hot cores (Charnley 1997; Hatchell et al. 1998). Overall, it is widely accepted that in star forming regions the formation of S-bearing molecules is largely determined during the cold collapse phase, when atomic sulphur freezes out on grains and probably forms H2S. When the protostar starts to heat its environment, H2S evaporates and it reacts with hydrogen atoms to give sulphur atoms. S rapidly reacts with OH and O2 to form SO, that in turn gives SO2by reacting with OH (e.g. Charnley 1997).
In this paper, we present large scale maps of several transitions of SO, SO2 and H2S in the molecular cloud L1689N, a molecular
cloud located in the Ophiuchi cloud complex at 120 pc from the
Sun (Knude & Hog 1998). Based on atomic oxygen observations,
Caux et al. (1999) found that the gas temperature in this
cloud is (
) K and the H2 density is larger than
cm-3. L1689N harbors two young protostellar
sources. The first one is IRAS 16293-2422 (hereinafter IRAS 16293), a
Class 0 protostar (15
)
still in the accretion phase
(Ceccarelli et al. 2000a; Walker et al. 1986; Narayanan et al. 1998; Zhou 1995). Like many other young protostars, IRAS 16293 is a
binary system with a total mass around 1.1
(Looney et al. 2000), whose two sources are separated by 5'',
namely a projected separation of 600 AU. The structure of the
envelope surrounding IRAS 16293 has been reconstructed based on
multifrequency H2O, SiO, O and H2CO line observations
(Ceccarelli et al. 2000a,b). In the outer region
(
AU), the envelope gas shows molecular abundances typical
of cold molecular clouds. In the inner region (
AU,
i.e. about 2'' in diameter) the abundances of H2O, SiO and H2CO jump to abundances typical of the hot cores around massive
protostars. This structure has been recently confirmed by
Schöier et al. (2002), who modeled the continuum and the line
emission from several other molecules. The second protostar, 16293E,
is a recently discovered low mass and very young Class 0 source
situated South-East of IRAS 16293. It was detected first by
Mizuno et al. (1990) as a strong NH3 peak emission. Its
protostellar nature is discussed in Castets et al. (2001, hereinafter
CCLCL01).
The L1689N region is complex and has long been known to house multiple
outflows
(Wootten & Loren 1987; Hirano et al. 2001; Fukui et al. 1986; Mizuno et al. 1990). The recent work by CCLCL01 claims that the two
protostars IRAS 16293 and 16293E drive three bipolar outflows. Two of
them originate from each of the two components of IRAS 16293, while the
third outflow probably emanates from 16293E
(Fig. 1). In the present article we will adopt the
scheme outlined in CCLCL01 (Fig. 1), but our main conclusions are
substantially unaffected by the actual evolutionary stage (pre-stellar
or protostellar) of 16293E, questioned in
Lis et al. (2002). What is important in the following
discussion is the presence in L1689N of at least one protostar, IRAS 16293, and six regions which shows an enhancement of SiO and/or H2CO emission, and that are sites of shocked gas marked as E1, E2,
HE1, HE2, W1 and W2 (Fig. 1). In
particular, CCLCL01 found that the brightest site of SiO emission, E2,
does not show up any H2CO enhanced emission, whereas the brightest
H2CO emission site, E1, is also accompanied by strong SiO emission.
HE2 and HE1 represent a third class, for only H2CO emission is
detected there and no SiO. The goal of the present work is to study
how the abundances of S-bearing molecules change in all these sites,
compared with the abundances in the IRAS 16293 protostar and in the
cloud.
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Figure 1: Sketch of the region seen face on. The dark grey outflows have been clearly identified by CCLCL01. Light gray used for the outflow emanating from IRAS 16293 and the W2 source indicates that these are only assumptions. |
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The article is organized as follows. The observations are presented in Sect. 2, the results are presented in Sect. 3, and the column density determinations are detailed in Sect. 4. In Sect. 5 we discuss the results, i.e. the observed changes of SO, SO2 and H2S abundances with respect to the previously measured SiO and H2CO abundances and what this may teach us.
Table 1:
Observed molecules together with the observing
parameters. The critical densities (
)
are computed for
temperatures between 50 and 100 K using the collisional coefficients
quoted in Sect. 4. The area mapped (
)
in each transition and the sampling of the map are
reported in the last two lines.
is the beam efficiency
of the telescope. The symbol "-'' implies that only observations
towards the "key'' positions have been obtained.
In June 1997 we obtained a map covering
in the SO2 molecular line with the IRAM-30m telescope. In November 2001, May and September 2002 we performed additional IRAM observations
of the SO2, 34SO and H2S
lines at some specific positions in L1689N, namely the shocked regions E1, E2, W1, W2 and HE2 (see Fig. 1 and the Introduction), the two protostars IRAS 16293 and 16293E, and the molecular cloud (at the
position
,
). In
the following we will refer to these eight positions as the "key''
positions. The SEST telescope was used in July 1998 to map an area
covering
in the SO molecular lines and
to map a smaller area covering
in the
H2S molecular transition. Table 1 summarizes the
different observed molecular transitions, together with their
frequencies and upper level energies.
All data were obtained in the position switching mode with an OFF position located at
,
from the center of IRAS 16293. Using the
frequency switching mode we checked that this position is free of any C18O and 13CO emission. Even though our observations are not
Nyquist sampled, we smoothed our data to the largest beamsize when
necessary, which is accurate enough, except in the case of
pathological source morphologies. Both with the IRAM and SEST telescopes, pointing and focus was checked every 2 h, using
planets, maser sources and strong extragalactic continuum sources. The
pointing corrections were found to be always smaller than 3'' and 5'' with the IRAM and SEST telescopes respectively. Polynomial
baselines of order 3 or less have been subtracted from the spectra.
The observing parameters for both telescopes are listed in
Table 1. In the following, all intensities will be
given in main-beam brightness temperatures. Below we give some
information which are specific to the IRAM-30m and SEST observations.
IRAM observations: the
IRAM 30-m telescope is located at an altitude of 2920 m near
the summit of Pico Veleta in Southern Spain. The 34SO
,
SO2 and H2S molecular emissions were observed simultaneously with the 3, 2
and 1 mm SIS receivers respectively, available at the IRAM-30m. The
SO2 emission observed in June 1997 was observed simultaneously
with other molecular transitions not presented here. The image
sideband rejections of all receivers were always higher than 10 dB.
Typical system temperatures were in the range
K. The three receivers were connected to units of the
autocorrelator, set to provide spectral resolutions of 40, 40, and 80 kHz at 98, 136, and 217 GHz respectively. At those frequencies, the
velocity resolutions are of the order of 0.1-0.2 km s-1.
SEST observations: the SEST telescope is a 15-m
single dish millimeter telescope operated jointly by ESO and a
consortium of Swedish institutions. It is located at an altitude of
about 2450 m in La Silla (Chile). To obtain the SO and H2S maps shown here, we used the dual 3/1.3 mm receiver. System
temperatures were in the range
K. This
receiver was always connected to the high resolution acousto-optical
spectrometer available at SEST, which provides 2048 channel of 43 kHz
width giving a 86 kHz spectral resolution. At the frequencies
considered here, that corresponds to a velocity resolution of 0.1-0.2 km s-1, comparable to that of the IRAM data.
Figure 2 shows the velocity-integrated intensity maps of the
SO
,
SO
and SO2
lines. The spectra (not smoothed) of
the six transitions of Table 1 observed towards the
"key'' positions are displayed in Fig. 3, and the
relevant line parameters are reported in Table 2.
The SiO
observations, published by CCLCL01, have
been added in
Fig. 3 and in Table 2 for comparisons.
![]() |
Figure 2:
Integrated intensity maps of SO
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The main-beam brightness peak temperature
,
and the linewidth
(FWHM) of Table 2
were estimated
from a Gaussian fit to the whole line profile, even in the case where
profiles are double-peaked.
We also estimated the width of the equivalent Gaussian with the same
integrated and peak intensity as the observed line and found that the
two methods differ by less than 30% with the exception of the SO
line in W1 which has an equivalent width of 2.5 km s-1 (versus 1.3 km s-1 in Table 2).
On the contrary, the velocity-integrated
intensity is the area integrated in the interval of velocities at the
"zero-intensity'' level. The velocity
is the velocity
at the peak or at the absorption dip for double-peaked profiles. They are
roughly the same for each observed transition and position, with
the exception of E2(HVC), whose emission is red-shifted by
6 km s-1.
The first remarkable result is that, against probably naive expectations, the shocked regions are not necessarily associated with evident enhancements of the emission of the observed SO, SO2 and H2S lines. For example, the E2 region, the brightest SiO 2-1 emission peak (see CCLCL01) is undetected in SO2, and only marginally detected in the lowest SO transition.
The second result is that IRAS 16293 is the only site where all
the molecular lines have been detected, including the high-energy
transition
.
Quite interestingly, we find that not only the
line profiles are broader than in the cloud but their widths tend to
increase with the upper energy level of the transition. We will
discuss in the Sect. 5 the implication of this behavior.
We now discuss in more detail the results toward the eight "key'' positions.
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Figure 3:
Spectra (not smoothed) of SO
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Table 2:
Line parameters (
(K),
v (km s-1),
(km s-1),
(K km s-1))
for the various transitions observed at the "key'' positions: the
two protostars IRAS 16293 and 16293E, the shocked regions E1, E2, W1, W2
and HE2, and the reference position in the cloud. The SiO observations were previously published in CCLCL01.
When no signal is
detected, we give an upper limit for the intensity equal to 3 RMS
of the relevant spectrum. The parameters are taken from the spectra
not smoothed.
The symbol "-'' implies that the relevant transition has not been
observed or detected.
The SO
emission is relatively bright (
)
all over the
cloud (Figs. 2 and 3). All the line profiles are
relatively narrow (with linewidths ranging between 1 and
)
and
peak around the systemic cloud velocity at
.
The absorption dip around the systemic velocity seen towards IRAS 16293 is probably due to the absorption by foreground material, as
also seen in the H2CO transitions (Loinard et al. 2001). On
the other hand, the absorption dip seen in the 34SO towards W1 may point to a very large SO column density in that direction or two different kinematic components. The contrast of the
velocity-integrated SO molecular emission at the various positions in
the cloud is relatively low: the emission peaks towards IRAS 16293 but
otherwise it is widespread in the region between W1 and E2
(Fig. 2). Remarkably, there is not a much brighter SO
emission towards the shocked regions with respect to the molecular
cloud. The only two clear exceptions are represented by E1 and E2,
where a high velocity component appears in the low energy SO transition (Fig. 3). In the following we will analyze
separately the E2 low velocity component centered at
(hereinafter E2(LVC)), and the E2 high velocity component at
(hereinafter E2(HVC)), because the two components can be clearly
disentangled. On the contrary, in E1 the situation is less clear and
hence we didn't pursue a separated analysis of the two components. We
emphasize that the E2(HVC) component is detected at more than 3 RMS
all along the red lobe of the outflow and it is also observed in the
SiO transitions (CCLCL01). This high velocity component is not seen
in SO
probably because of unfavorable excitation
conditions. Indeed, we will show in the next section that the gas
density in E2(HVC) is lower than in the ambient gas (see
Table 3). This fact, already recognized by CCLCL01,
leads to the supposition that the E2(HVC) may represent the wind
shock, whereas the lower velocity component, E2(LVC), may trace the so
called cloud shock (Hollenbach 1998). Finally, we also see
hints of red and blue wings originating from the molecular outflows
driven either by IRAS 16293 or 16293E in the direction of W1, E1 and HE2 whereas no wing is seen in W2.
In the higher energy line SO
the emission is peaked towards
IRAS 16293 and W1 (cf. Fig. 2) while the emission from the
cloud is, as expected, weak. Relatively strong emission is also
detected around E1. Moreover, the SO
spectra in those positions
have pronounced red and blue wings. The same happens in HE2 where the
SO
spectrum shows clearly the presence of a blue wing. The
emitting region around IRAS 16293 has a characteristic size of
namely it is not resolved in
.
Finally, the observed SO spectra towards 16293E are similar
to those in the cloud: narrow, intense in the SO
transition and
weak in the SO
transition, and no signs of wings are
evident. Overall, 16293E does not seem to have any enhanced SO emission compared with the cloud.
Unlike the SO
,
the SO2
emission is limited to a few spots
(see Figs. 2 and 3), specifically toward IRAS 16293, E1 and W2, probably because of the excitation conditions
(the critical density of the SO2
transition is about 20 times the
SO
critical density). Unfortunately, because of the lack of
other observations around W2 we have no idea of its extent.
Remarkably, no significant SO2 emission is detected towards E2.
SO2 has also been detected at the cloud position.
The linewidth (1.4 km s-1)
is equivalent to that of N2H
- a good tracer of
extended, cold and quiescent gas - observed at the same position (CCLCL01).
Probably because of similar critical densities, the SO2
and
SO
emission shows up in the same regions. In addition,
with the exception of E1, the
spectra show characteristics
similar to the SO
spectra, like for example the linewidth, in all
"key'' positions. On the contrary, in E1 the SO line profiles are
characterized by a strong ambient component with blue- and redshifted
wings of much lower brightness, whereas the high-velocity component
appears as bright as the ambient component in the
line, so
that the whole SO2
linewidth is approximately twice that of the SO
.
IRAS 16293 is the only position where H2S
has been detected. The
line is relatively bright and rather broad (
). There is no emission detected in shocks nor in the ambient
cloud. Conversely, the lower H2S transition, observed only
toward IRAS 16293, 16293E and the cloud, has been detected in all these
positions with a double-peaked profile towards the protostars
and possibly the cloud position. The absorption dip is likely due to the
optical depth of the line. In fact, Minh et al. (1991) have
observed the same H2S transition towards several star forming regions
and found an opacity of
10 for similar column densities.
Contrary to the sulphur bearing species, the emission of SiO
is stronger in the shocked regions than towards IRAS 16293. The spectra in E1
and W1 show high velocity wings much more marked than for the SO and SO2 lines. The high velocity component of E2 seen in SO
is very strong in SiO
and have similar
(
)
and
(
)
for these two transitions.
In this section we estimate the column densities of the observed species, namely SO, SO2 and H2S, as well of the SiO and H2CO previously observed by CCLCL01, in the "key'' positions. In order to do that, we first estimate the density and temperature of the gas in each position. To derive the gas temperature and density, as well as the column densities of the different species, we used the theoretical predictions from an LVG (Large Velocity Gradient) model described in detail in the next paragraph. The next three paragraphs describe the derivation of the gas temperature and densities as well as the column density of the observed species in the cloud, in the shocked regions, and in IRAS 16293 respectively. Note that we did not carry out the analysis on 16293E, because the observed emission in that position seems to be dominated by the cloud emission.
To derive the gas temperature and density, as well as the column
densities, we compared the observed line intensities with the
theoretical predictions from a LVG model, which self-consistently
accounts for the excitation conditions (density and temperature) as
well as the line opacities. For the escape probability
we
used the following function of the line optical depth:
,
valid in the case of a homogeneous
and isothermal semi-infinite slab. More details can be found in
Ceccarelli et al. (2002). Note that the dust emission is neglected
in the present computations.
The spectroscopic data of SO, SO2, H2S, SiO and H2CO are all
taken from the JPL catalogue
(http://spec.jpl.nasa.gov/ftp/pub/catalog/catform.html;
Pickett et al. 1998). The collisional coefficients are from
Green (1994) for SO, Palma (1987) for SO2, Turner et al. (1992) for SiO, and
Green (1991) for H2CO respectively. The collisional
coefficients of H2S are not available in literature, and we
estimated them from the collisional coefficients of H2O, multiplied
by a factor 5 (following the discussion
in Turner 1996). This is a very rough approximation,
giving rise to unfortunately very rough estimates of the H2S column
density. Furthermore, the collisional coefficients used for the other
molecules are not available in the full range of temperatures probed
by our observations. In particular the low temperature regime is often
missing. In this case we have extrapolated the coefficients at the
lowest available temperature with a
law.
Finally, our code considers the first 50 rotational levels of each
molecule, unless the relevant collisional coefficients are available
for a lower number of levels.
Table 3:
Gas temperature and density, and column densities of SO, SO2, H2S, SiO and
in six "key'' positions.
The column densities are beam-averaged in the shocked regions (on 27''
for SiO, 26'' for H2CO, 24'' for SO, 18'' for SO2 and 25'' for H2S),
whereas they are corrected from the beam dilution in IRAS 16293.
Errors on the estimates are around 15% if considering only the statistical
uncertainties. However, a factor two of uncertainty has to be considered,
when the uncertainty on the density and temperature of the emitting
gas is considered.
To estimate the gas density and temperature, and the SO column density in the cloud reference position we simultaneously best-fitted the 34SO and 32SO
observations (that
give a line opacity for 32SO
), and the 32SO
and
observations
(whose ratio, together with the 32SO
intensity, gives pairs of gas temperature and density
values). Note that we used the elemental ratio
34S/
(Chin et al. 1996; Wilson & Rood 1994; Lucas & Liszt 1998),
and we assumed that the emission fills up the beam, when comparing
observations obtained with different telescopes. The result of the
modeling is shown in Fig. 4. We found that the ensemble of
our observations constrains the SO column density in the cloud to be
in the range 3 to
cm-2. Adopting the gas
temperature of 26 K, as found by Caux et al. (1999), we
derive a density of
.
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Figure 4:
Cloud reference position modeling.
Theoretical predictions as function of the gas temperature and density for
two different SO column densities. The curves refer the values of
three observed quantities taking into account the uncertainties.
Solid lines: observed integrated intensity ratio
SO
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The column densities of the other species, namely SO2 and H2S,
have been derived assuming the same density and temperature. The SO2 abundance was found to be
,
a value equivalent to the ones
obtained in other clouds (see Table 4). In the
case of H2S we cross-checked the column density derivation with
LTE computations, because of the rough approximation of the
collisional coefficients. For this we assumed an excitation
temperature of 5 K, as found by Swade (1989) and
Hirahara et al. (1995) in similar molecular clouds (L134N and TMC-1) for several molecules whose fundamental transitions have
A-coefficients similar to that for the H2S
transition. In fact Minh et al. (1989) used an
excitation temperature of 5 K for the H2S
transition to compute the column density of H2S in the
molecular clouds L134N and TMC-1, which have temperatures and
densities similar to L1689N. Note that increasing the excitation
temperature by a factor two, would decrease the H2S column
density by the same amount. We assumed a H2S ortho-to-para ratio
equal to 3. Finally the SiO and H2CO column densities are taken
from the modeling by Ceccarelli et al. (2000b, 2001) respectively
. The column densities
derived for all species are reported in Table 3 and the
abundances in Table 4.
Table 4:
Abundances in molecular clouds. References: L1689N: SO, SO2 and
H2S from this work, SiO from Ceccarelli et al. (2000b) and H2CO from Ceccarelli et al. (2001); L134N:
SO and SO2 abundances from Swade (1989), H2S
from Minh et al. (1989), SiO from
Ziurys et al. (1989),
from
Ohishi et al. (1992); TCM-1: SO abundances from
Hirahara et al. (1995) and Pratap et al. (1997), SO2from Irvine et al. (1983), H2S from
Minh et al. (1989), SiO from Ziurys et al. (1989),
from Ohishi & Kaifu (1998).
We have used the gas temperature and density derived by
CCLCL01 in E1, E2(LVC), E2(HVC), W1,
W2, i.e. in the
shocked regions, by comparing the SiO observed lines (J from 2 to 5)
with the relevant LVG model. The uncertainty in the derived densities
and temperatures is around a factor two. We did not try to derive
independently the gas temperature and density for each molecule,
because of the lack of enough usable transitions. In fact, of the two
SO transitions, the lowest lying transition is dominated by the cloud
emission so it cannot be used to probe the shocked gas, and only one
transition of SO2, H2S and H2CO respectively has been
detected. We therefore derived the column densities of those last
four species assuming the same density and temperature derived from
the SiO observations. This is an approximation that does not take
into account the possible structure of the shocked gas, but the
results are indeed not much affected by this assumption. In principle
each molecular species may originate in slightly different physical
conditions and hence the computed column density may be consequently
mis-evaluated. In practice, though, the error associated with this
approximation is lower than about a factor two.
For example, Lis et al. (2002), using H2CO transitions,
found different values for the temperature
(45 K) and density (
cm-3) in E1.
Even taking those values, the derived column density
of the four molecules listed in Table 3 wouldn't change
by more than a factor two with respect to those quoted in the table.
Finally, we also assumed that all the used lines are
optically thin (e.g. Blake et al. 1994). The column
densities, averaged on the relevant beam, derived for all species in
each of the four shocked sites are reported in Table 3.
Unfortunately, since we don't have observations of the H2S lowest
transitions in the shocks, the column density of this molecule is very
poorly constrained.
As mentioned in the Introduction, the envelope of IRAS 16293 is formed
by (at least) two components: an inner core characterized by a high
temperature and an outer cold envelope with a temperature close to the
cloud temperature. The hot core is a small region (2'' in
diameter) whose temperature is about 100 K and the density
(Ceccarelli et al. 2000a; Schöier et al. 2002). At
that temperature the grain mantles evaporate injecting into the gas
phase their constituents
(Ceccarelli et al. 2000a,b). The outer envelope is
more extended (
20'') and colder, with a temperature of 30 K (a
little larger than the L1689N temperature) and a density of
(Ceccarelli et al. 2000a; Schöier et al. 2002). The
molecular abundances in the outer envelope are similar to those in the
molecular cloud, with the possible exception of the deuterated
molecules. On the contrary, in the inner core, several molecules,
those believed to be released from the grain mantles or formed from
those evaporated molecules, undergo a jump in their abundances. In
the following we focus on the abundance of the S-bearing molecules in
the inner hot core. Blake et al. (1994), using multifrequency
observations, constructed the rotational
diagrams of SO and SO2 and derived rotational temperatures
relatively large (
80 K). It is therefore very likely that the
bulk of the emission for both molecules originates in the inner core.
Furthermore, all linewidths are larger than 3 km s-1 and increase with
the upper level energy of the transition, once again
arguing for an inner core origin.
Pursuing this hypothesis we computed the column densities using these
rotational diagrams, corrected
for the beam dilution, reported in Table 3. In
estimating the H2S column density we assumed that the
line emission originates entirely in the
hot core, as it is suggested by its observed relatively large
linewidth. Using then the estimates of the H2 column density in
Ceccarelli et al. (2000a) (
)
we
derive the abundances reported in Table 5. Note that,
provided that we use our volume density in the Schöier et al. (2002)
model, our derived abundance compare extremely well with their
abundances (which, as a result, is multiplied by a factor five),
supporting the validity of our method.
Actually Schöier et al. found
abundances a factor 5 lower than ours, because they used a factor
five larger density than us in the
inner core. The two density estimates differ because of the different
diagnostics used to derive them: Schöier et al. used
the continuum spectral energy distribution, whereas
Ceccarelli et al. used the water line spectrum. When
CO lines are used instead, Schöier et al. found the
same density in the inner region (Schöier private communication).
We use therefore the estimate by Ceccarelli et al. (2000a), but keep
in mind that we may be overestimating the abundances by a factor 5.
Finally, the SiO and
column densities are taken from the
Ceccarelli et al. (2001,2000a,b)
modeling.
Table 5:
Abundances of SO, SO2, H2S, SiO and H2CO with
respect to H2 in the hot core of IRAS 16293.
The abundances of SO, SO2 and H2S are estimated by the present work
observations, assuming a H2 column density of
(Ceccarelli et al. 2000a).
The SiO and H2CO abundances are taken from Ceccarelli et al. (2000a,b).
References of the other sources:
L1157-mm (IRAS 20386+6751): Bachiller & Perez Gutierrez (1997);
Orion-KL: SO, SO2, SiO and H2CO from Sutton et al. (1995),
H2S from Minh et al. (1990);
G10.47, G29.96, G75.78, G9.62, G12.21, G31.41 and G34.26:
Hatchell et al. (1998); BF-Class 0:
Buckle & Fuller (2003).
The abundance ratios between the observed species are reported in Tables 4-6. They show variations up to two orders of magnitude. In the following we analyze in detail the variations associated with the cloud, the shocked regions and the protostar IRAS 16293 respectively.
Adopting the H2 column density derived by
Caux et al. (1999) by means of atomic oxygen observations
in L1689N,
cm-2,
and using the column densities derived in the previous section
(Table 3) we obtain the abundances reported in
Table 4. In the table we also report, for comparison, the
abundances measured in L134N and TMC-1, two among the best studied
molecular clouds. With the possible exception of formaldehyde, which
seems to be underabundant, L1689N has abundances typical of other cold
molecular clouds and it is therefore interesting to study how those
abundances change in the region either because of the presence of
shocks or the presence of a protostar (IRAS 16293).
Table 6: Abundance ratios in the shocked regions and in the cloud, as derived from the column densities of Table 3. References for other outflows: L1157-B1 and L1157-B2: Bachiller & Perez Gutierrez (1997); CB3: Codella & Bachiller (1999); L1448, B1 and CepA: Martin-Pintado et al. (1992); NGC 2071: Chernin & Masson (1993).
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Figure 5: The abundance ratios observed in the different shocked regions and in the molecular cloud and presented in Table 6: SO/H2CO, SO2/H2CO and SiO/H2CO on the lower panel and SO2/SO, SiO/SO2 and SiO/SO on the upper panel. The arrows represent upper and lower limits of the SO2/SO, SiO/SO2, SO/H2CO and SiO/H2CO ratios respectively. |
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In the shocked regions, with the exception of E2, it is practically
impossible to derive the absolute abundances of the observed
molecules, because of the difficulty to derive reliable estimates of
the H2 column densities. In fact, usually the H2 column
densities are estimated from the CO millimeter observations converting
the measured CO column densities into H2 column densities. The
method relies on the capacity to disentangle the contribution of the
cloud from the shocked gas in the CO emission. However, this is
feasible when the shocked gas emits at relatively large velocities,
i.e. when the two contributions can be separated based on their
spectral properties (see for example Bachiller & Perez Gutierrez 1997).
In the specific case of the outflows in L1689N, the projected velocity
of the shocks is too small and the cloud and shocked gas cannot be
disentangled, except in the E2 position, where a high velocity
component is present. In order therefore to estimate the H2 column density in the shocked regions we would need high spatial
resolution observations, because single dish CO measurement are totally
dominated by the cloud emission. For this practical reason, we
computed only abundances ratios, as summarized in
Table 6. Note that the quoted ratios have been obtained
dividing column densities averaged on different beam sizes (18'' for
SO2 observations and 25'' for the others), so that the
SO2 ratios may be off by a factor two (in the shocked regions).
Figure 5 shows a graphic representation of the derived
abundances ratios in the shocked regions and in the cloud.
The first result to note is that, with the exception of the SO2/H2CO ratio, all other ratios in the shocked regions differ by more than a factor two when compared to the ratios in the cloud. In particular, the largest variations, by up a factor 100, are shown by the ratios with the SiO, confirming that SiO is the best molecule to trace shocks (Garay et al. 2002; Schilke et al. 1997; Caselli et al. 1997).
A second robust result is that SO2 is enhanced with respect to SO
in the E1, W1 and W2 shocks, when compared to the cloud (by up a
factor 10). This also confirms a rather general theoretical
prediction that SO2 is overproduced with respect to SO in the
shocked gas, because of endothermic reactions that lock most of the
gaseous sulphur in SO2 on timescales relatively small 103 yr (e.g. Pineau Des Forêts et al. 1993). Remarkably, SO2/SO is
lowest in the strongest SiO shock of the region, E2. CCLCL01 argued that E2 is an older shock based on H2CO and SiO observations compared
with models predictions and this would agree with the low SO2/SO ratio, as SO2 is expected to be converted into atomic sulphur at
late stages
yr (e.g. Hatchell et al. 1998).
The third result, is the constant SO2/H2CO ratio, within a
factor 2, in the shocked regions E1, W1 and W2, and in the cloud. Why
SO2/H2CO has the same value in these shocked regions and in
the cloud seems a remarkable coincidence for which we do not have an
explanation. On the contrary, the constant SO2/H2CO ratio
in these shocked regions may be due to the timescales involved in the
formation and destruction of the two molecules, even though the
mechanisms of SO2 and H2CO formation in shocks are expected to
be different. On the one hand, formaldehyde is thought to be formed
onto the grain mantles, released into the gas phase because of the
mantle sputtering in the shock, and finally converted into more
complex molecules by gas phase reactions on timescales larger than
yr (Charnley et al. 1992). SO2 follows a
totally different route: it has been proposed that sulphur is released
into the gas phase mainly as H2S, following also mantle sputtering,
and then H2S is transformed into SO and SO2 via neutral-neutral
reactions on timescales of
103 yr
(e.g. Charnley 1997). Later on
yr, gaseous sulphur is expected to be in atomic sulphur.
Therefore, the constant SO2/H2CO ratio in E1, W1 and W2 may
tell us that these three shocks have similar ages, around 104 yr if
models are correct, whereas E2 is a much older shock,
yr, and SO2 and H2CO both disappear because transformed
into S and complex O-bearing molecules respectively.
The overall emerging picture is that the combination of the variations
in the abundance of different molecules can in first instance give an
estimate of the age of the shock. Table 7 summarizes the
situation. The times noted in the Table 7 are really
modeled dependent and are meant to represent a sequence. The
simultaneous presence of SO, SO2, H2S, H2CO, and SiO would
mark relatively young shocks (104 yr), whereas the only
presence of SiO emission would point towards relatively older shocks
(
yr).
Table 7: Proposed schema of the formation and destruction of SO, SO2, H2S, SiO and H2CO as function of time. The times are indicative and are meant to represent a sequence.
Finally, comparison with other molecular outflows can only be very approximate, but still somewhat illustrative (Table 6). The only molecular outflow at our knowledge in which SO, SO2, SiO and H2CO abundances have been measured is the L1157 outflow (Bachiller & Perez Gutierrez 1997). Looking at Table 6, L1157 B1 and B2 are rather similar to E2 with respect to the ratios involving SiO, i.e. they are relatively enriched in SiO with respect to the other shocks of L1689N and the molecular cloud. The likely interpretation is that E2, B1 and B2 are all relatively strong shocks. Yet, contrary to what happens in E2, H2CO, SO and SO2 are detected in L1157 B1 and B2, which would point towards relatively young shocks (Table 7). Note though that SO seems to be underabundant with respect to H2CO and SiO in B1 and B2 when compared not only to E2, but also to E1, W1 and W2. At the same time, SO2 is overabundant. Whether this is because of the stronger shock in B1 and B2 that converts SO into SO2 more quickly, or because the shocks in L1157 have a different age of E1, W1 and W2, is impossible to say at this stage. A larger statistics on a larger number of outflow system is necessary to better understand this point.
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Figure 6:
Theoretical predictions of the
evolution of SO2/SO and H2S/SO ratios as a function
of time. The curves are adapted from the Hatchell et al. (1998)
model, that assumes a gas temperature of 100 K and a density of
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Table 5 quotes the absolute abundances of SO, SO2, H2S, H2CO and SiO in the hot core of IRAS 16293 and in the molecular cloud. As evident from the table, the sulphuretted molecule abundances exhibit a strong enhancement with respect to the molecular cloud abundances. The abundance of SO increases by a factor 200, SO2 by a factor 1300 and H2S by a factor 1700 (see also Schöier et al. 2002).
As we mentioned in the Introduction, the sulphur species are expected
to be useful to evaluate the age of hot cores. We could not resist
the temptation to compare the measured abundance ratios with published
theoretical predictions. In Fig. 6 we compare the H2S/SO
and SO2/SO ratios obtained in the IRAS 16293 hot core with the
theoretical values predicted by chemical models. The observed H2S/SO and SO2/SO ratios are consistent with the theoretical
predictions by Hatchell et al. (1998), but differ substantially
with those by Charnley (1997). Both models use the
sulphur chemistry described in Charnley (1997) and assume
that the bulk of sulphur is stored in iced H2S onto the grain
mantles. When the dust temperature exceeds 100 K, the grain mantles
evaporate releasing the H2S into the gas phase, which is slowly
converted into SO and subsequently into SO2 by neutral-neutral
reactions. The only difference between the two models is that
Hatchell et al. use an updated rate for the reaction
of H2S with atomic hydrogen. This difference substantially changes
the evolution of the abundances of H2S, SO and SO2 at times
larger than about 104 yr. Our observations support the
Hatchell et al. (1998) model, as shown in Fig. 6. The
agreement between observations and model predictions for an age of
yr is remarkable, as independent estimates of the
age of IRAS 16293 converge towards the same value
(Ceccarelli et al. 2000a). On the same figure we also report the
values observed towards L1157-mm. Based on these observations and on
the model predictions, L1157-mm has about the same age of IRAS 16293.
We caution that the error bars are indeed large enough to make the
agreement between observed and predicted values just a
coincidence. In this sense, this result may just mean an
encouragement to pursue this kind of studies on a large sample of low
mass protostars, where a statistical trend could be established. We
have indeed started such a study on a sample of low mass protostars,
whose we have derived the physical structure (i.e. density and
temperature profiles) and formaldehyde abundance profiles
(Maret et al. 2002).
A direct comparison between the measured abundances in the hot cores quoted in Table 5 and that of IRAS 16293 can only be done with respect to Orion-KL, as in the other sources the abundances are not corrected for the beam dilution (Hatchell et al. 1998; Buckle & Fuller 2003). With respect to Orion-KL, the hot core of IRAS 16293 is enriched in SO, SO2 and H2CO, whereas it is deficient in H2S. The most plausible explanation is that the different abundances reflect a different composition of the ices (from which H2CO and H2S evaporate), a fact now well documented by the study of deuterated molecules in high and low mass protostars (e.g. Ceccarelli 2002). However, the different gas temperature and density may also play a role. Finally, the SO2/SO and H2S/SO ratios of IRAS 16293 differ by about a factor 100 and 10 respectively with respect to the relevant ratios measured in the high mass protostars of the Hatchell et al. (1998) sample. Again, the difference may be attributed to both different evolutionary stages and different gas densities and temperatures.
Following the theoretical expectations that gaseous sulphur in the
warm gas is mainly locked into SO, SO2 and H2S
(e.g. Charnley 1997; Pineau Des Forêts et al. 1993), while silicon
is mainly locked into SiO (Herbst et al. 1989), the
(SO+SO2+H2S)/SiO ratio gives a measure of the gas phase
elemental sulphur over silicon abundance ratio in the IRAS 16293 hot
core. This value is about 300, to compare with the solar elemental
sulphur over silicon abundance ratio of 0.5. It is unlikely that we
overestimate the overall quantity of sulphur. Conversely, it could
have been underestimated because of a possible optical thickness of
the sulfuretted molecular lines and/or the presence of other important
sulphur-bearing molecular species not considered here. Thus, the
measured (SO+SO2+H2S)/SiO ratio suggests that there is an
important deficiency of silicon in the warm gas of the IRAS 16293 hot
core. The likely explanation is that silicon is mostly depleted into
the non-volatile, refractory cores grains, whereas sulphur is
depleted mostly onto the volatile grain mantles. This conclusion
supports the thesis by Ruffle et al. (1999) that sulphur and
silicon follow different routes of depletion, and specifically that
sulphur is depleted at the time of mantle formation. Another
possibility to explain this deficiency of silicon is that silicon is
mainly in another form than SiO in the warm gas such as Si, Si+or SiO2. The atomic silicon is difficult to observe, since it
has its ground transition at 129 m, i.e. a wavelength range
obscured by the atmosphere. Observations obtained with the Long
Wavelength Spectrometer on board ISO did not detect any signal larger
than about
erg s-1 cm-2(Ceccarelli et al. 1998), corresponding to an upper limit for the
Si column density of
cm-2, and an abundance
lower than
,
i.e. about 500 times more than the SiO abundance found in IRAS 16293. For Si+, it is unlikely that the
UV field in the observed regions is strong enough to ionize the
silicon.
Finally, the total abundance of SO, SO2 and H2S in the IRAS 16293 hot core is
,
namely more than ten times
lower than the corresponding solar abundance
.
Even adding up the OCS, whose abundance is about
that of SO (Schöier et al. 2002),
the overall sulphur abundance is still
low. Unfortunately (Schöier et al. 2002) could not estimate
the CS abundance in the hot core of IRAS 16293, but CS is unlikely
to be the main reservoir of sulphur.
The other possible reservoir of sulphur, the atomic sulphur
(Charnley 1997), is extremely difficult to observe,
because the ground state transition is at 25
m, i.e. in
a wavelength range obscured by the atmosphere.
We have presented a quantitative observational study of the most important S-bearing molecules, namely SO, SO2 and H2S, in the region of L1689N. We derived the column density of these molecules plus SiO and H2CO molecules in six regions of L1689N: the cloud, the young protostar IRAS 16293, and four shocked regions.
We found that SiO is the molecule that shows the largest abundance
variations in the shocked regions, whereas S-bearing molecules show
more moderate variations. Remarkably, the region of the brightest SiO emission in L1689N, namely E2, is undetected in SO2, H2S and H2CO and only marginally detected in SO. We argued that this is
possibly due to the relatively old age (
yr) of
this shock.
In the other weaker SiO shocks, SO2 is enhanced with respect to SO, in
agreement with theoretical expectations that predict the conversion
of the gaseous sulphur mostly into SO2 on timescales of 103 yr. In the same regions, the SO2/H2CO ratio is of order of
unity. We argued that this may point to relatively young shocks (
104 yr), where SO2 has already formed and H2CO has not
yet destroyed.
Putting together the observed combinations of the SO, SO2, H2CO and SiO ratios, we proposed a schema in which the different molecular ratios correspond to different ages of the shocks.
Finally, we found that SO, SO2 and H2S have significant
abundance jumps (200, 1300 and 1700 respectively) in the inner hot
core of IRAS 16293. We compared the measured abundances with
theoretical models and discussed the derived protostar age. However, we
cautioned that a more detailed study is necessary to draw reliable conclusions.
The hot
core of IRAS 16293 seems to be enriched in SO, SO2 and H2CO with
respect to Orion-KL, probably because of a different initial
composition of the ices in the two sources. Comparing the
SO+SO2+H2S/SiO ratio in the hot core of IRAS 16293, we found
that silicon is largely deficient in the warm gas (by a factor 600), supporting the thesis that silicon is depleted into the grain
refractory cores whereas sulphur is depleted into the grain volatile
mantles. Nonetheless, sulphur in the IRAS 16293 warm gas is also
deficient.
Acknowledgements
We thank the IRAM and SEST staff in Pico Veleta and La Silla for their assistance with the observations, and the IRAM and ESO Program Committee for their award of observing time. We would like to thank G. Fuller, the referee, for useful comments.We are grateful to A.G.G.M. Tielens and M. Walmsley, for helpful discussions on sulphur and silicon chemistry. V. Wakelam wishes to thank F. Herpin for his help on data reduction.