S. Nikolic1,2 - L. E. B. Johansson1 - J. Harju3
1 - Onsala Space Observatory,
439 92 Onsala, Sweden
2 -
Astronomical Observatory,
Volgina 7, 11160 Belgrade 74, Serbia, Serbia and Montenegro
3 -
Helsinki University Observatory,
Tähtitorninmäki, PO Box 14, SF-00014 University of Helsinki, Finland
Received 29 April 2003 / Accepted 17 July 2003
Abstract
We have mapped the dense parts of the cometary-shaped, star-forming dark
cloud L 1251 in the rotational lines of HCN, HNC, HCO+ and CS at 3 mm, and
observed selected positions in SO, CH3CCH and rare isotopomers of the mapped
molecules. Using the CS line we detected 15 cores with sizes of
0.1-0.3 pc. New estimates of the fraction of dense gas in the cores
yield a revised average SFE of
10%. Although 3 times lower than the
previous estimate, this high SFE still points to externally triggered star
formation in the cloud. Around IRAS 22343+7501, the source proposed to drive a
previously detected extended CO outflow, our data suggest the existence of
either a rotating HCO+ disk or a dense outflow with a dynamical age of
years. A stability check seems to rule out the disk
interpretation. We suggest that both continuum sources of Beltrán et al. are
protostars each driving its own outflow.
Using methyl acetylene as a
thermometer we find indications that at lower temperatures the A and Especies are defined by different partition functions. A "temperature
gradient'' was found in the cloud, with the highest temperature detected in the
head region. The column density ratios derived from these observations and the
previously published NH3 data show in general little variations, but for two
exceptional locations. One of these is in the tip of the "head'' with high
relative SO and NH3 abundances, and the other is in the "tail'' with low CO
and HCO+ column densities with respect to HNC, HCN and NH3. In the first
case the abundance ratios can probably be explained by an advanced stage of
chemical evolution assisted by an elevated temperature. The second location is
likely to be an example of CO and HCO+ depletion, and the implication is
that also HNC and HCN belong to the molecules which are more resistant against
freezing-out than CO and HCO+.
Key words: ISM: abundances - ISM: clouds - ISM: molecules - ISM: individual objects: L 1251
The overall star forming efficiency (SFE, defined as the ratio
/(
+
), where
is the total gas mass of a
molecular cloud and
is the mass of embedded protostars) for the
Galaxy is estimated to be only
2% (Myers et al. 1986). Similar
low SFEs are typical for dark clouds where low - mass stars are born.
However, there are clouds with much higher - than - average SFE which
indicates that triggered star formation is significant in such cases. For
example supernova shock fronts may stimulate the formation of stars through
their local effects on density, kinetic temperatures, turbulence, ionization
degrees and, as a result, chemical processes.
L 1251 (Lynds 1962) is an example of a dark cloud with an estimated SFE
as high as 30% (Kun & Prusti 1993; KP). Both location and cloud
morphology suggest that external triggering has contributed to the on - going
star formation. This cometary - shaped cloud, at a distance of 300 pc
(KP), lies on the Eastern boundary of the Cepheus cloud complex, with the
"head'' turned towards the center of the Cep - Cas Void (Grenier et
al. 1989). At least two supernovas have exploded in this area within
the last 106 years, as indicated by the presence of the major
radio - continuum loop, Loop III (Berkhuijsen 1971) and a
runaway star HD203854, whose space velocity suggests that it might have been a
companion of a supernova some
-106 years ago (Kun et
al. 2000). The Cep - Cas Void is suggested to be created by a
third SN (Grenier et al. 1989). However, the estimated age of
104 yr means that this SN is much too young to have affected the star
forming processes observed now.
Detected H
stars in the vicinity of the cloud (Kun 1982) and
seven embedded YSO (KP) indicate that the cloud is an active low - mass
star formation site. Of the YSOs, IRAS 22376+7455 and IRAS 22343+7501,
classified as Class I YSOs (Mardones et al. 1997) apparently power
two detected CO outflows (Schwartz et al. 1988;
Sato & Fukui 1989; Sato et al. 1994). Herbig-Haro
objects (Balázs et al. 1992; Eiroa et al. 1995) and H2O masers (Tóth & Walmsley 1994;
Wilking et al. 1994; Xiang & Turner 1995; Claussen et
al. 1996; Tóth & Kun 1997) are observed in their
vicinity.
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Figure 1:
The 13CO integrated intensity map of L 1251 in the
(-2, -6.5) km s-1 velocity range obtained by Sato et
al. (1994). The center position is
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Figure 2:
HNC (upper panel) and HCN (lower panel) integrated intensity
maps of L 1251 in the (-2, -6.5) km s-1 velocity range (for HCN the velocity
range of the main 2-1 component). The center position is the same as in
Fig. 1 and the
observed positions are indicated by dots. The intensity scale is in
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The overall distribution of molecular gas in L 1251 has been previously studied by Sato et al. (1994) and Tóth & Walmsley (1996; TW). Sato et al. identified five C18O cores which they designated as "A" to "E" in increasing RA direction (see Fig. 1). In their ammonia survey, TW discovered three regions of dense gas, "head", "north" and "tail"; the "head" region containing 3 ammonia cores (H 1 to H 3) and the "tail" region consisting of 4 cores (T 1 to T 4), whereas in the "north" a single ammonia core was detected (N, see their Fig. 3). The ammonia "head" group of cores corresponds to core "E" of Sato el al., group "north" to core "C" and the "tail" group to core "A".
Apart from NH3, commonly used tracers of dense material are CS, HCO+, and the isomeric molecules HCN and HNC, because their rotational transitions near 3 mm are easy to observe and have critical densities higher that 105 cm-3. In this paper we present maps of these molecules in the densest parts of L 1251, and estimate their column densities along with some other molecular species in selected positions. The cloud contains both protostellar and prestellar condensations and shows clear signs of external influence. The physical conditions are therefore likely to vary across the cloud. The aim of this study was to investigate whether any indications of such variations can be traced in the observed molecular lines, and how these possible changes are reflected in chemical abundances.
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Figure 3: The CS (2-1) integrated intensity map of L 1251. The velocity range of the emission, the center position, the intensity scale and markers are as in Fig. 1. Contours start from 0.65 to 1.3 by 0.13 K km s-1 and from 1.3 to 2.34 by 0.26 K km s-1. |
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Figure 4: The HCO+ (1-0) integrated intensity emission map of L 1251 for the (-2, -8) km s-1 velocity range. The center position, the intensity scale and markers are as in Fig. 1. Contours start from 0.8 and increase by 0.45 K km s-1. |
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We used the Onsala Space Observatory's (OSO) 20-m telescope over five
observing sessions in 1998, 1999 and 2000 to map the cloud in the
HCN (1-0), HNC (1-0), CS (2-1) and HCO+ (1-0) transitions. Selected
positions were subsequently observed in 13CO (1-0), C18O (1-0),
H13CN (1-0), HN13C (1-0), C34S (2-1),
H13CO+ (1-0), SO (21-11) and CH3CCH (5K-4K). The
receiver was a SIS mixer with a typical
K (SSB) in the frequency
range used. We used a 1600-channel correlator with 20 MHz bandwidth (i.e. a
velocity resolution of 0.04 km s-1 at 90 GHz). The HPBW of the
telescope at 90 GHz is 45
and the main beam efficiency is 0.6. The
pointing was checked by observing several SiO maser sources and we estimate
the pointing uncertainty to be about 3
rms in Az and El. The
observations were made either in the frequency (main isotopes) or the dual beam
switching mode (rarer isotopes). The chopper-wheel method was used for
the calibration, and the intensity scale is given in terms of
.
We used
mostly a grid point spacing of 30
,
however, occasionally a 60
step was used. The data were reduced using the XS
package.
Table 1: Derived parameters from the CS (2-1) observations. Sizes and masses assume a distance of 300 pc.
Our HCN and HNC (1-0) integrated intensity maps are shown in Fig. 2, and the corresponding CS (2-1) and HCO+ (1-0) maps are shown in Figs. 3 and 4, respectively. The area mapped in HNC and HCN covers the two ammonia cores in the "head'' designated as "H1" and "H2" by TW, the northern part of L 1251, and the cores "T1", "T2" and "T3" in the tail. All these ammonia cores are associated with newly born stars or protostars. The maps in CS and HCO+ include also the starless core "H3'' on the western side of the head core group. Also indicated in these figures are the locations of YSOs and T Tau stars probably associated with the cloud. Labeling of these sources in the upper panel of Fig. 2 follows that of Table 3 of KP.
A comparison between our maps and the NH3 map of TW reveals two major differences: i) in NH3 the head and tail regions have similar integrated intensities whereas in HCN, HNC, CS and HCO+ the head region is clearly brighter. For the latter molecules the higher integrated intensities in the head are due to a larger number of velocity components in the line of sight (see Sect. 3.2); ii) the core around IRAS 22343+7501 (N1a) is not visible in NH3. Ammonia peaks further up in the north near the T Tau star #9.
The maps show local maxima which roughly correspond to the ammonia cores detected by TW. Because of the denser sampling and the higher spectral resolution available in the present study we see, however, more structure than discernible in the previous NH3 maps. Using the spatial - velocity information available, we have identified altogether 15 cores in the mapped region, most of which can be seen in all four lines. Some of the NH3 cores of TW divide in our maps into two components. Following the nomenclature of TW we label the five cores studied in detail as H1a, H2a, H2b, N1a and T1a, where "a" and "b" indicate a presence of a secondary peak or a separate velocity component not resolved in the previous NH3 observations.
Table 1 lists the identified cores in order of decreasing RA offset
(i.e., in head-to-tail direction). The columns are: (1) core
identification number; (2) line center velocity; (3, 4) core center in the RA
and Dec offsets with respect to the center of the map; (5) full width at
half intensity of the integrated emission corrected for the beam size; (6) the full
halfwidth of the global line profile of the core; (7) virial mass; (8)
mass calculated from C18O and (9) association with YSOs or T Tau stars
and core designation following TW. The core size, D, is estimated from
the extent of the half power intensity contour deconvolved with the beam
assuming Gaussian shapes for the beam as well as for the source. In most cases cores
are elliptical, and we use the geometrical mean of the major and minor axes to
define the size. Virial masses are derived using the formula
(see Johansson et al. 1998). For 5 cores
independent mass estimates are derived from the C18O observations as
,
where
and
are source diameter and beam size at the corresponding frequency,
respectively (see Nikolic et al. 2001).
We have calculated standard deviation of the core diameters for all available
molecules to be in the range 0.1 to
0.3 pc. The emission extents
of observed molecules agree, in most cases, within
of the
arithmetical mean of a core.
The large differences between masses estimated from the virial theorem and the C18O data, obvious in three cores, may possibly be linked to the presence of young stellar objects; enhanced turbulence and/or ordered motions like, e.g., outflows could cause the discrepances.
From the C18O data, Sato et al. (1994) derived a total gas mass in
the head region of 65 .
Adding up our estimates of core masses in the
same region, we arrive at
75
(using
where
available, otherwise
), indicating that most of the mass is concentrated in
the dense cores. For the northern region of L 1251 Sato et al. (1994)
estimate a total gas mass of 56
.
We derive a mass of 36
for
core 7 (i.e. the N1a core) implying that for this region at least 65% of the
total mass is in the form of dense gas.
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Figure 5:
The HNC channel velocity maps of the "head'' region. The observed
positions are indicated by dots. The intensity scale is in
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The observed emission in the head covers the range from -6 to -2 km s-1, while the northern area and the tail are seen only in the ranges (-6, -4) and (-5, -3) km s-1, respectively. This is consistent with the dominant velocity components of the 13CO emission according to Sato et al. (1994).
Figure 3 shows the HNC (1-0) line emission of the
head region in four velocity channels. The other observed lines show similar
features in the corresponding channel maps. Two velocity components, centered
at -3.5 km s-1 and at
-4.5 km s-1 are clearly
seen in the maps. These two velocity components are also discernible in the
HN13C spectra of H1a, H2a and H2b.
Close inspection of the HCO+ spectra in the N1a core area reveals a possible
interaction with the CO outflow (Sato & Fukui 1989) within
from the IRAS 22343+7501 source. At the estimated distance of
the cloud this equals 36000 AU. In this area the line shapes of HCO+ show
significant wing emission between -8 and -2 km s-1 as well as
self-absorption features. Figure 6 shows the extents of the blue-
and red-shifted emission at the most extreme velocities. The structure presented
mimics either a rotating HCO+ "disk'' or a "toroid'' (see, e.g., Torrelles
et al. 1983) around the protostar, or a dense HCO+outflow. The dashed line in Fig. 6 gives roughly the orientation of
the CO outflow axes. If the structure is considered to be a disk, then the disk
radius is
10 000 AU. Such large HCO+ "disks'' around low - mass
protostars are not uncommon (see, e.g. Fridlund et al. 2002). On the
other side, if we are observing a dense outflow, then the dynamical age is
approximately the same for both wings and is equal to
years.
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Figure 6:
Integrated intensity map of the blue - (greyscale) and the
red-shifted (contours) HCO+ emission of the N1a core. The velocity ranges
are (-7, -8) and (-2, -3) km s-1 for the blue and the red wings,
respectively. The intensity scale is in
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Table 2:
Total column densities derived using the LTE assumption (see the
text). For the three cores in the "head'' appropriate velocity components were
used (see Table 1). The formal errors are 10-15%, which include
intensity calibration (10%) and spectral noise.
Table 2 gives column densities, derived assuming LTE conditions
and optically thin emission, towards selected positions. In addition, it was
assumed that the excitation temperature,
,
is 10 K for
13CO and C18O, and 6 K for the other species which are likely to be
subthermally excited (see, e.g., Caselli et al. 2002). The column densities of C34S and HN13C show the
largest variations across the cloud. The C34S column density has a minimum
towards H1a in the head, whereas HN13C peaks towards T1a in
the tail. The column densities of the other molecules change less than by a
factor of three. A comparison between different molecules brings forth some
pairs with large variations in the column density ratios, and others with
minor changes. For example, H13CO+/C18O and HN13C/H13CN
are roughly constant (
and
2, respectively),
C34S/SO has by far the lowest value towards H1a, and HN13C/C18O
is clearly largest towards T1a.
Table 3:
H2 total column densities (in [1021 cm-2]) obtained
from 13CO using a conversion factor of
N (H2)/N (
(Dickman & Clemens 1983) and from C18O using the ratio
[C18O]/[H2] = 1.7
(Frerking et al. 1982),
and the calculated fractional abundances of the main isotopomers with respect
to [H2] derived from [C18O].
We have estimated also the H2 column densities,
,
with the
aid of C18O and the conversion factor
determined by Frerking et al.
(1982). The
values have been then used to derive
the fractional abundances of other observed molecules. In the conversion to
the main isotopomers fractional abundances the following isotopic ratios
characteristic of the local ISM have been used:
and
(Wilson & Rood 1994). The H2 column density estimates and
the fractional abundances are given in Table 3. According to this table the
position H1a has the lowest CS abundance and the largest SO abundance. The
fractional HCN and HNC abundances seem to peak towards T1a. N1a, with the
highest H2 (in fact C18O) column density, has the lowest SO abundance.
In the other molecules the changes are less marked. It should be noted that the
fractional abundances derived here reflect column densities relative to
C18O, and do not represent the true relative abundances with respect to
H2 in case the C18O/H2 column density ratio changes, e.g., due to
CO depletion.
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Figure 7:
The observed CH3CCH 5-4 spectra smoothed to a velocity resolution
of 0.12 km s-1. Core designation is given in the upper right corner and
the observed K=0,1,2,3 transitions are marked with solid lines. The
intensity scale is
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Symmetric top molecules, like methyl cyanide, CH3CN, and methyl acetylene,
CH3CCH, make good temperature probes because each K-component of a given
J rotational transition has a different energy level. Radiative
transitions between different K-ladders are prohibited by the selection rule
;
thus the populations of different K-ladders are determined by
collisions and depend mostly on the gas kinetic temperature. Also, being
relatively close in frequencies, all K components can be observed
simultaneously, avoiding calibration problems.
The J=5-4, K=0,1,2,3, transitions of methyl acetylene were observed
towards the previously selected five cores in L 1251.
Figure 7 shows the spectra. To estimate kinetic
temperatures we have used the rotational diagram method (see, e.g., Anderson
et al. 1999) which assumes LTE conditions and optically thin
emission. The derived rotational temperatures are unexpectedly high as are the
associated errors. The reason for this can be traced back to the intensity of
the K=0 relative to the K=1 transition; with the exception of the region
H1a, the observed ratios all indicate
K. Such results
can be explained in terms of non - LTE excitation or that the total
abundances of the A (K=0) and E (K=1) species are not equal.
Assuming
K, the E/A population ratio would be about
1.5 based on the K=0 and 1 transitions. Askne et al. (1984) find
that statistical equilibrium and rotational diagram calculations agree, with
the exception of cold regions. Their analysis indicate that the total
abundances of the A and E species are equal, however, defined by
different partition functions at low kinetic temperatures. For TMC - 1,
their observations show an intensity ratio of
1 between the K=0 and 1,
J=5-4 transitions, while the statistical equilibrium analysis indicates
K and
K. Observationally, this is very
similar to our sample (with the exception of H1a), possibly indicating that
this head core is warmer than the rest of the cores observed here.
Table 4: The results of the CH3CCH rotational diagram method weighted with observational errors plus E, A species uncertainties (case I) and using the K=1 and K=2 lines only (case II). Ammonia temperatures are from TW.
To proceed we have introduced an uncertainty in the A and E species populations of 30%. The results are given as "case I'' in Table 4 and clearly show smaller errors in the derived parameters in spite of larger total errors in the input data, a reflection of the inconsistent intensities of the K=0 and 1 transitions. Figure 8 shows graphically this inconsistency common for all our cores with the exception of H1a. In "Case II'' we have only used the K=1and 2 transitions (i.e. the E-species) to estimate the rotation temperatures. These results are further discussed in Sect. 6.4.
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Figure 8: Rotation diagram of the CH3CCH 5-4 transition for core N1a. For definition of case I (dotted line) and case II (solid line) see the text and Table 4. The error bars refer to the observational uncertainties only. |
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Our high spectral resolution observations have revealed two distinct velocity components in the head region, separated by about 1 km s-1. The same components are most likely present in other molecules as well. The fact that these were interpreted as velocity gradients by TW is due to their cruder velocity resolution.
Similarly, a superposition of these two components at least partly explains the observed CS line asymmetry, i.e., "the infall profiles" seen by Mardones et al. (1997) towards IRAS 22376+7455. Yet a possible infall of dense gas cannot be ruled out.
Five out of six embedded YSOs, as proposed by KP in the region we have
surveyed, are most likely associated with the denser parts of L 1251: FIR sources
#4, #5 and #6 in the tail area (associated with the cores 15, 13 and 11 in
Table 1, respectively), source #8, (IRAS 22343+7501) in the northern area
with core 7 (N1a) and source #14 (IRAS 22376+7455) with core 4 (H2b).
Additionally, based on their projection onto dense areas, three T Tau
candidate stars are probably associated with the cloud: sources #16 and #17
in the topmost part of the head area and source #9 in the northern area of the
cloud, all coincident with detected H
emission stars. Sources #13
and #15 in the head area are both blended by the strong IR emission from
IRAS 22376+7455, and have ill-defined error ellipses. Since the former source
does not seem to be associated with any dense core it may be either only
projected on the cloud area or is a faint, low - mass embedded star (Kun,
priv. com.). The position of source #15 correlates well with the center of
core 3 (see Fig. 5), where the HCN peak integrated
emission is
2.5 K km s-1. Yun et al. (1999) derived an
80% likelihood of tracing an embedded Class 0 YSO if the detected HCN
emission is stronger than 3 K km s-1. Thus, source #15 may be an
embedded Class 0/I YSO.
To derive the SFE of the cores we assume
for all the embedded
YSOs and T Tau stars with unknown masses, i.e., sources #6, #15, #16 and
#17. IRAS 22376+7455 and IRAS 22343+7501 have estimated masses of
and
,
respectively (Kun 1998). If we use the
masses of the cores listed in Table 1 (
), SFEs for the
H1a, H2a, H2b, N1a and T1a cores are approximately 11%, 9%, 18%,
6% and 20%, respectively. On average, SFE of the observed cores would be
13%, almost 3 times lower than the previously estimated SFE for the
whole cloud, but still 5-6 times higher than the overall SFE for the Galaxy.
Based on IRAS and sub-mm continuum observations Mardones et
al. (1997) derived the bolometric temperature of IRAS 22343+7501
to be 108 K, and classified the source as a Class I YSO. Using Chen
et al. (1995) empirical relation between age and bolometric temperature for
YSOs with
K, we estimate IRAS 22343+7501 to be
years old. This age would classify the source as a very
young Class I YSO. Sato et al. (1994) derived the dynamical timescale (using
an inclination of
)
of the CO outflow to be
and
years for the blue- and the red-wing, respectively. Provided
that IRAS 22343+7501 is the CO outflow driving source, consistent age
estimates would require that the flow plane is close to the plane of sky, i.e.,
tilted by 4-8
.
Kun (1998) estimated the mass of a hypothetical
central star to be
.
Near - infrared images
(Rosvick & Davidge 1995) and recent 3.6 cm and 6 cm VLA
continuum observations (Grissom Meehan et al. 1998; Beltrán et
al. 2001) show that this IRAS source actually consists of several
protostellar objects. The near - IR images exhibit a 20-30
large
nebulosity corresponding to a maximum size of 9000 AU. VLA continuum
measurements revealed two sources, separated by 7
,
i.e.,
about 2000 AU. Both continuum sources have spectral indices consistent with
thermal emission, and any of them could be the CO outflow driving source
(Beltrán et al. 2001). The proposed HCO+ disk would encompass
both sources.
Under the assumption that the properties of the HCO+ emission (see
Fig. 6) are similar to those in the N1a core, i.e.,
,
we derive a total mass of this
feature as
[
], where IMB is
the velocity integrated emission [K km s-1] and A is the area of the
HCO+ wing emission [pc2]. It is further assumed that the isotopic
abundance ratio is 77 and that the fractional abundance of HCO+ is
(see Table 3). The total wing emission integrated over
the full velocity range and wings areas is 0.1 K km s-1, yielding
5
of molecular gas, equally distributed between
the approaching and receding emission regions.
Assuming that this emission originates from a disk rotating with a velocity of
2 km s-1 at its outer edge we find the centrifugal force to be
12 times larger than the gravitational force of the central star and the "disk''
combined. Since the observed radial velocity is only a lower limit to the
"rotation velocity", this factor (proportional to
)
can most
likely be considered as a lower limit, although our derived mass could be
underestimated if the HCO+ is more saturated than in the surroundings. However,
the H13CO+ spectrum toward the IRAS source (N1a core center) shows no signs
of wing emission, indicating a saturation level of the main isotope emission
similar or lower than assumed. Based on our high ratio of centrifugal to
gravitational forces we thus find it unlikely that the observed HCO+ wing
emission defines a disk.
On the other hand, if the observed structure represents a dense outflow, the
derived dynamical age of the flow is
years (not corrected for an
unknown inclination), i.e., an order of magnitude less than the CO outflow. The
discrepancy between the dynamical ages for the CO and the suggested HCO+ outflow
indicates that they are of different origin. This is further emphasized by the
apparently different orientations of the flows in the plane of the sky as well as
in the radial direction (in contrast to CO, the red- and blue-emission regions
of HCO+ partly overlap). As noted earlier, we can probably treat the derived
mass of
5
in the HCO+ wings as an upper limit. We define a
lower limit by assuming optically thin HCO+ wing emission and arrive at
0.5
.
Using this range of masses, the released kinetic energy of
the HCO+ flow is estimated to be 4-
J and mechanical luminosity
to be 1-
J/s i.e., 0.03-0.3
;
the lower limit comparable
to the mechanical luminosity of the CO outflow although the CO outflow is spread
over an area of the order of a magnitude larger. In this picture of two outflows
traced by the CO and the HCO+ emission, both continuum sources of Beltrán et al.
(2001) could be protostars each driving its own outflow.
According to gas - phase chemical models the main route for production of
HCN and HNC is by dissociative recombination with electrons (e.g. Hirota et
al. 1998):
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(1) |
In conclusion, our HCN/HNC and CH3CCH results favor a kinetic temperature in
excess of 25 K towards H1a and less or significantly less than
20 K for the rest of the cores. This temperature increase of the
dense gas is likely caused by shocks originating from outflows and/or
radiation from the embedded stars. We searched the 2MASS All Sky Catalog and in
the head region of L 1251, clustered around the two H
stars and the
two embedded YSOs (#14 and #15), we found 23 point sources. This suggests
that the head region of L 1251 is much more productive than the northern core
(8 point sources), and the tail cores (6 point sources).
Regarding the possibility of external heating, the newly born pre-main
sequence stars observed by KP could, in principle, heat the outer parts of the
cloud to 20-25 K. However, there the gas density is too low to excite the
molecular transitions observed by us. Another possibility, as suggested in the
Introduction, is that the cloud has encountered at least one external
shock; however such shocks have cooling times in a dense gas of the order of years
(see, e.g., Smith & Rosen 2003).
Based on the production pathways via ion-molecule or neutral-neutral reactions, and their dependence on neutral carbon (C I), some molecules are classified either as "early-time'' (104-106 years after the onset of chemistry) or "late-time'' species (maximum abundances reached at steady state, after about 106-108 years of chemical evolution; e.g. Herbst & Leung 1989). Of the molecules discussed here HCO+, SO and NH3 are usually considered as late-time molecules; HCO+ because it is formed from CO, and the latter two because their formation mechanisms involve relatively slow neutral-neutral reactions. SO is furthermore destroyed primarily by C I which at later stages is locked up in CO (e.g. Nilsson et al. 2000). CS forms early, but its abundance remains roughly constant because of recycling via HCS+(Nejad & Wagenblast 1999). The situation of HCN and HNC is less clear in this picture. The precursor ion, HCNH+, is produced mainly via a reaction between NH3 and the C+ ion, thus involving both a typical "late-time'' molecule and an ion characteristic of young chemistry.
In the recent models of Rawlings et al. (2002) it is shown that in an "H-rich'' environment the nitrogen chemistry is initiated at an early stage, and because of the less effective destruction of He+ (due to a low CO abundance), NH3 forms quickly thereafter. Under these circumstances also HNC is produced at early times via C + NH2. NH2 forms from a dissociative recombination of NH3+, which is one of the precursors of ammonia. In the "H-poor'' models of Rawlings et al., on the other hand, the formation of HCN is efficient via N + CH2 or N + CH3 at early times. From these results one can expect that at early stages of chemical evolution the HCN/HNC abundance ratio depends strongly on the initial H content.
The division of molecules into "early-time'' and "late-time'' species is valid eventually only until a protostar is formed in a core, as this may change the physical conditions and consequently the chemical composition of the ambient cloud via heating of dust grains, enhanced turbulence and radiation field (see, e.g. Nejad et al. 1990). As a consequence of intensified desorption and ionization, we are then able to observe characteristically "young'' chemistry.
X - ray surveys of star forming regions showed that Class I-III YSOs have
significant X - ray emission (e.g., Getman et al. 2002; Casanova et
al. 1995). The heating effect of this emission is very localized
(e.g., Lepp & McCray 1983), but the X - ray induced ionization
affects the whole core/cloud. Casanova et al. (1995) derived
X - ray induced ionization rate throughout the
Oph cloud core to be
comparable with the usually assumed cosmic rays ionization rate of
10-17 s-1. A paradoxical situation may occur in the sense
that a dynamically older core, with a central Class 0 embedded YSO is
chemically younger than a pre-protostellar core (Kontinen et al. 2000).
All the dense cores studied here are associated with protostars or newly born stars:
the presence of T Tau stars in the vicinity of H1a would indicate a dynamical age
106-107 years for this core, while the embedded YSOs (#6, #8, #14 and
#15) in the remaining 4 cores point to ages of 104-105 years, with the tail
core being the youngest. The fact that H1a is located near the compression
front corroborates the notion that the core is the most evolved among the
L 1251 cores.
The fractional abundances given in Table 3 and the column density ratios
given in Table 5 show some differences but lack clear trends with
respect to the adopted dynamical ages of the cores. The large SO/CS and NH3/CS
column density ratios in H1a in the head conform with the idea that this core has
reached an advanced stage of evolution. However, as discussed above one would possibly expect the presence of
a T Tau star to alter the chemistry towards "younger'' stages. This suggests that
the influence of this T Tau star is hardly significant, provided that present
chemical networks of dense and cold clouds predict the evolution of molecular
abundances in a rather accurate way. On the other hand, the relatively low CO
and HCO+ column densities in T1a with respect to NH3 and HNC (see Table 3) can be
understood in two alternative ways: first, T1a can be in an early stage and CO and
HCO+ have not yet had time to reach the steady state abundances. This alternative
would then imply that NH3 and HNC are here early time species, and according to
the models of Rawlings et al. (2002) would indicate an initially
"H-rich'' environment. The second alternative, and in fact the more likely, is that CO
and HCO+ are depleted in T1a. Previous observations have namely shown that
NH3 can remain in the gas-phase in the situation where CO is heavily depleted
(e.g., Willacy et al. 1998; Tafalla et al. 2002). These studies concern
mainly starless cores, but may be valid for cores with low-mass protostars which
have not affected their surroundings yet. The high fractional HNC and HCN abundances
in T1a suggest furthermore that also these molecules are more resistant against
freezing-out than CO and HCO+. This may be attributed to the replenishment of
the HNCH+ ion via the reaction between NH3 and C+. For the rest of the cores, Table 5 shows no definite trends which
allow age sequencing. At the very best, our analysis indicates a possibility to
discriminate cores with dynamical ages 105 years from those older than
106 years. However, to fully establish such a conclusion, a considerably
larger sample of cores is needed.
Table 5: Ratios of column densities, based on Table. 3. Relative errors of the ratios are in the range of 5-15%, for SO, CS, HCO+ and HNC. For ratios that include ammonia, errors are in the range 20-25%.
We have completed a survey of dense cores in L 1251 in several high gas density tracers: HCN, HNC, CS and HCO+. On the larger scales, all observed molecules have similar distributions, including that of the previously published NH3 data. Velocity components observed are consistent with the 13CO data by Sato et al. (1994). The "head'' part of the cloud consists of two gas components, whose central velocities differ by about 1 km s-1. Altogether 15 dense cores can be identified in our maps.
Around IRAS 22343+7501, which is proposed to power the extended CO outflow, we
have detected HCO+ wing emission, the distribution of which is resembling
either a rotating disk or a dense outflow. Stability considerations seem to
exclude a disk interpretation. If an outflow, the derived dynamical age and
apparent orientations suggest that its origin is most likely different from
that of the CO outflow. In the direction of IRAS 22343+7501 Beltrán et
al. (2001) have detected two continuum sources, separated by
,
having spectral indices consistent with thermal emission. Thus,
both sources could be protostars each driving its own outflow.
We have made additional observations towards five cores in SO, CH3CCH and rare isotopomers of the mapped molecules in the selection. Using methyl acetylene and the HCN/HNC ratios as thermometers, we find a "temperature gradient'' in the cloud. The highest temperature is detected in the head region.
The derived column density ratios do not change much from core to core. This is probably traceable to the fact that all cores are star forming and most of the molecules observed are characteristic of mature chemistry. Two of the cores have, however, peculiar abundance ratios. The core located in the tip of the "head'' of the cloud, has clearly higher SO/CS and NH3/CS ratios than seen anywhere else in the cloud, suggesting that the core has reached a very late stage of chemical evolution, probably assisted by an elevated temperature due to shock - heating in the head. In the other exceptional core, lying in the more quiescent "tail'' of the cloud, the column densities of CO and HCO+ are low compared with those of HNC, HCN, and NH3. We suggest that this is due to depletion of CO and HCO+, and as a corollary, that also HNC and HCN, like NH3 belong to the molecules that remain longer in the gas phase than CO and HCO+. The similar behavior of the former three molecules can probably be explained by their close relationship in the ion - molecule reaction schemes.
Four out of the five cores considered in L 1251 have embedded protostars in
different stages of evolution. The revised average SFE of 10 % is
almost 3 times lower than the previous estimate, but still
5 times higher
than the overall SFE of the Galaxy. This high SFE indicates a contribution from
externally triggered star formation.
Acknowledgements
The research was partly funded by the Ministry of Science and Technology of Serbia grant No. P1191 (2001-2004) and by the Finnish Center for International Mobility (CIMO). We are very grateful to Dr. M. Kun for the suggestion to use the 2MASS ASC, and to Dr. N. Mizuno who provided the 13CO data presented in Fig. 1.Onsala Space Observatory is the Swedish National Facility for Radio Astronomy and is operated by Chalmers University of Technology, Göteborg, Sweden, with financial support from the Swedish Natural Science Research Council and the Swedish Board for Technical Development.