Using the IRAM 30-m telescope, we observed molecular absorption lines from c-C₃H₂ produced in diffuse clouds toward the high-mass star forming regions W51 e1/e2 and W49N to determine the abundance ratio between the cyclic and linear isomers of C₃H₂ (N_c/N_l). The abundance ratio is found to be 3-5 in the sources where l-C₃H₂ was previously detected. A possible source of uncertainty in the determination of N_c/N_l is related to the estimate of N(c-C₃H₂). The main goal of this paper is verification of this hypothesis

Gerin, M.

Kulczak-Jastrzębska, M.

Lis, D.

English

Caltech Authors - Main

A Comparison of c-C₃H₂ and l-C₃H₂ in the Spiral Arm Clouds

ACTA ASTRONOMICA
Vol. 62 (2012) pp. 313–321
A Comparison of c-C3 H2 and l-C3 H2 in the Spiral Arm Clouds
M. K u l c z a k - J a s t r z e˛ b s k a1 , D. L i s 2 and M. G e r i n3
1 Astronomical Observatory of the Jagiellonian University, 30-244 Orla 171, Kraków,
Poland
e-mail:kulczak@oa.uj.edu.pl
2 California Institute of Technology, Cahill Center for Astronomy and Astrophysics
301-17, Pasadena, CA 91125, USA
e-mail:dcl@caltech.edu
3 LERMA, UMR 8112 du CNRS, École Normale Supérieure and Observatoire de Paris,
24 Rue Lhomond, 75231 Paris cedex 05, France
e-mail:maryvonne.gerin@lra.ens.fr
Received August 29, 2012
ABSTRACT
Using the IRAM 30-m telescope, we observed molecular absorption lines from c-C 3 H 2 pro-
duced in diffuse clouds toward the high-mass star forming regions W51 e1/e2 and W49N to deter-
mine the abundance ratio between the cyclic and linear isomers of C 3 H 2 (Nc/Nl ). The abundance
ratio is found to be 3–5 in the sources where l-C 3 H 2 was previously detected. A possible source of
uncertainty in the determination of Nc/Nl is related to the estimate of N (c-C 3 H 2 ). The main goal
of this paper is verification of this hypothesis.
Key words: Radio lines: ISM – ISM: molecules – ISM: abundances
1. Introduction
Carbon is the fourth most abundant element in the interstellar medium (ISM),
and also the most versatile for building molecules. Carbon participates in numer-
ous chemical reactions at any temperature, from the very cold dense cores, to warm
and hot gas. Of the over 150 molecules detected in interstellar and circumstel-
lar environments about 75% have at least one carbon atom, while one fourth are
hydrocarbons. Moreover the heaviest and most complex molecules are organic
molecules with carbon. This statistic does not take into account the Polycyclic Aro-
matic Hydrocarbons (PAHs), nor the Diffuse Infrared Band (DIB) carriers, which
are more likely large organic molecules (Herbig 1995). While CH, CH+ and CN
have been known in the ISM since the 1940, the detection of other molecules in
the diffuse gas has decreased afterwords. However recent, deep observations have
314 A. A.
revealed and confirmed the presence of diatomic, triatomic and even more com-
plex molecules in the diffuse gas. Recent studies have shown that the C-rich dif-
fuse interstellar medium is more chemically active than previously thought, with
molecules as large as C3 (Goicoechea et al. 2004, Oka et al. 2003) and c-C3 H2
(Lucas and Liszt 2000, Gerin et al. 2011) (c-C3 H2 is the most widespread inter-
stellar ring molecule). The abundances of polyatomic molecules, such as c-C3 H2 ,
HCO+ , HCN and C2 H are somewhat higher than can be explained by conven-
tional chemical models (van Dishoeck and Black 1986). Small carbon containing
molecules, with a total of about 1 to 3 carbon atoms have several interesting prop-
erties: they show a widespread spatial distribution in the interstellar medium, they
likely participate in the formation of long carbon molecules and are involved in
photo-fragmentation process of larger species, such as PAHs. It is widely believed
that carbon chain or ring species might be candidates for the Diffuse Interstellar
Bands (Herbig 1975, 1995).
2. Observations
Absorption line studies toward strong background continuum sources are of
interest because they allow probing diffuse, low-density clouds. The determination
of the line opacity is independent of the excitation temperature if Tex ≪ Tc , where
Tc is the brightness temperature of the continuum source. Such absorption line
observations also allow measurements of column densities one or two orders of
magnitude lower than those obtained through emission line studies.
2.1. Cyclic and Linear Forms of C3 H2
Cyclopropelidene c-C3 H2 was the first cyclic molecule to be detected in the
ISM (Thaddeus et al. 1985, Vrtilek et al. 1990) and subsequently found to be ubiq-
uitous throughout the Galaxy, even in the diffuse gas (Cox et al. 1988). Its linear
isomer propadienlidene, H2 CCC, is the first stable member of the cumulene car-
bon chains, characterized by double bonds between the carbons and terminal non-
bonded electrons. Propadienlidene has also been observed in a number of sources,
albeit at a lower abundance. It was first detected by Cernicharo (1991) in the cold
dark cloud TMC1. The observed abundance ratio between cyclic and linear C3 H2
in TCM1 is 70 (Cernicharo et al. 1991). In the IRC+10216 circumstellar enve-
lope this ratio has a value of at least 30, whereas for the diffuse gas values lower
typically by a factor of ten have been derived.
2.2. Spiral-Arm Clouds
Cernicharo et al. (1999) detected linear C3 H2 in absorption from the spiral-
arm clouds seen on the lines of sight toward giant HII regions in the Galactic plane
(W49N, W51, Sgr B2). We have also studied some of these sources (Table 1).
Physical conditions in the spiral-arm clouds are supposed to overlap with those
Vol. 62 315
we inferred for the diffuse and translucent gas occulting extragalactic continuum
sources. For the spiral-arm clouds 13 CO emission is relatively strong and we con-
clude that they are more akin to the denser, darker, translucent objects studied by
Turner (1998). So the spiral-arm clouds toward W49 are at least 3–4 times higher
in column density than the most opaque of the clouds studied by Lucas (2000).
T a b l e 1
Coordinates and distances of the target sources
Source name α (J2000) δ (J2000) D [kpc] vLSR [km/s]
W51e1/e2 19h23m43.s9 +14◦30′25.′′9 7.0 56.0
W49N 19h10m13.s2 +09◦06′12′′ 11.4 8.4
2.3. Data Analysis and Processing
The observations were performed with the IRAM 30-m telescope in 2006 Au-
gust and December. We used the A100 and B100 receivers simultaneously with
either the A230, B230 or the C150, D150 receivers. All receivers were operated in
the single sideband mode, with a rejection of the image sideband better than a fac-
tor of 20. The observations were performed using the wobbling secondary reflector
to obtain flat and stable baselines and an accurate measurement of the 3 mm radio
continuum intensity. The wobbler throw was 240 ′′and the switching rate 0.5 Hz.
The weather conditions were average, with 5–10 mm of precipitable water vapor
in August 2006 and less than 3 mm in December 2006. The pointing was checked
on nearby planets and continuum sources, and was found accurate to within 5 ′′ .
Spectra were recorded with the flexible, high spectral resolution back-end VESPA,
tuned to the spectral resolution of 40 kHz and the spectral bandpass of 120 MHz,
covering a velocity range of 400 km/s. We also used the 1 MHz filterbank to obtain
broad-band spectra. We searched for the 21,2−10,1 transition of ortho c-C3 H2 at
85.338 GHz. The data processing was done using the IRAM GILDAS software
package (Pety 2005). The resulting IRAM 30-m spectra are shown in Figs. 1, 2
and 3.
The data were first calibrated to the T ∗A scale using the chopper wheel method
(Penzias and Burrus 1973), and subsequently converted to the main beam bright-
ness temperature scale using efficiencies appropriate for 85 GHz: Feff = 0.95 and
Beff = 0.75. The continuum intensity Tmb(c) was measured by fitting flat baselines
to the spectra. Line parameters of the detected absorption features: central opac-
ity τ , velocity v and line width ∆V, were derived by fitting Gaussian absorption
316 A. A.
Fig. 1. Spectra of c-C 3 H 2 toward W51 and W49N obtained with the IRAM 30-m telescope. The
velocity scale is given relative to the Local Standard of Rest (LSR). The spectra have been offset for
clarity
Fig. 2. Absorption spectra of c-C 3 H 2 toward W51 (the emission lines were removed). The source
continuum temperature was about 0.95 K
Vol. 62 317
Fig. 3. Absorption spectra of c-C 3 H 2 toward W49N (the emission lines were removed). The source
continuum temperature was about 1.95 K
T a b l e 2
Line Gaussian parameters of the c-C 3 H 2 absorption components toward W51e 1 e 2 and W49
Source name vLSR ∆V Tmb Tmb(c) τ
[km/s] [km/s] [K] [K]
W51e1e2 5.6 3.36±0.48 −0.13 1.16 0.12±0.04
45.6 1.06±0.14 −0.11 1.16 0.10±0.04
65.1 2.89±0.14 −0.43 1.16 0.46±0.06
W49N 15.2 1.54±0.14 −0.48 2.34 0.22±0.02
34.1 3.36±0.14 −0.24 2.34 0.11±0.02
39.5 2.38±0.14 −0.50 2.34 0.24±0.02
59.5 3.18±0.17 −0.35 2.34 0.16±0.02
62.9 2.54±0.13 −0.49 2.34 0.23±0.02
profiles to the spectra. The central opacity is defined as:
τ =− ln
(
1−
Tmb
J(Tex)− J(Tcmb)−Tmb(c)
)
(1)
where Tmb is the line temperature with respect to the continuum, J(T ) is the black
body function ( hνk )/(e
hν
kT −1) . The excitation temperature (Tex ) of the transition is
318 A. A.
typically not much higher then 2.7 K (the cosmic background radiation temperature
Tcmb ), since the densities of the regions where the lines formed are so low that the
molecules are not collisionally excited. Under this assumption the optical depth of
absorption line will be given directly from line to continuum intensity ratio as:
τ =− ln
(
1+
Tmb
Tmb(c)
)
. (2)
The line parameters derived from the IRAM observation of the c-C3 H2 transition
are listed in Table 2.
3. Results
We compute molecular column densities using the relation:
Ntot = Z(Tex) ·Nu ·
eEu/kTex
gu
= Z(Tex)
8piν3
c3
·
eEu/kTex
guAul · ehν/kTex−1
Z
τ · dυ (3)
where Z(Tex) is the partition function computed at the excitation temperature Tex ,
Ntot and Nu are the total column density and the column density in the upper state
of the transition, respectively, gu is the statistical weight of the upper level, Aul is
the Einstein spontaneous emission coefficient, ν is the line frequency and
R
τ · dυ
is the line opacity integrated over the line profiles. For a Gaussian line profile the
integrated line opacity is proportional to the product of the line full width at half
maximum ∆V · τ and the central opacity τ . The factor is very close to unity, and
can be written as
√
pi/4ln2 ≃ 1.065. The ortho c-C3 H2 column densities were
derived from relation of Lucas and Liszt (2000):
N(c-C3H2) = 4.36 ·1012 ·1.065 ·∆V · τ [cm−2]. (4)
The results of calculation are shown in Table 3. We compare our estimates of the
abundance ratio between the cyclic and linear isomer of C3 H2 with those found by
Cernicharo et al. (1991).
4. Discussion
Cernicharo et al. (1999) compared their column densities of l-C3 H2 with the
Madden et al. (1989) measurements of the (11,0−10,1 ) ortho c-C3 H2 rotational
transition at 18.343 GHz (excitation energy E/k of 0.9 K) toward high mass star
forming regions (W51e1 e2 , W49N, W51D and Sgr B2). In W51e1 e2 and W49N,
the 11,0−10,1 transition shows complex emission/absorption profiles where the
emission corresponds in velocity with the HII region itself, and absorption ap-
pears at velocities corresponding to intervening cold clouds. Toward W51e1 e2 ,
a narrow absorption feature at 66 km/s is contaminated by a broad emission fea-
ture (∆V = 12 km/s) of the same molecule at the source velocity. A similarly
Vol. 62 319
T a b l e 3
Results
Source name v∗LSR vLSR N(l-C3H2) N(c-C3H2)∗ N(c-C3H2) Nc/N∗l Nc/Nl
[km/s] [km/s] [cm−2] [cm−2] [cm−2]
W51e1e2 6.0 5.6 < 4.0E11 1.7E12 1.92E12 > 4.25 > 4.80
45.5 45.6 < 1.0E11 0.5E12 0.5E12 > 5.0 > 5.0
66.3 65.12 4.7E11 1.6E12 6.17E12 3.4 13.1
W49N 15.8 15.2 4.0E11 3.0E12 1.60E12 7.5 4.00
34.1 34.1 < 4.5E11 3.5E12 1.69E12 > 7.8 > 3.84
39.1 39.5 < 2.6E11 3.5E12 2.61E12 > 12.3 > 10.0
60.0 59.6 < 2.9E11 2.5E12 2.38E12 > 8.6 > 8.20
63.0 62.9 < 2.2E11 2.8E12 2.71E12 > 12.7 > 12.3
∗Cernicharo 1999
complicated spectrum of c-C3 H2 at 18.343 GHz is observed between 0–20 km/s
toward W49N region. The line absorption profile contaminated by the emission
feature could be a source of error because the contamination would increase the
value Nc/Nl . This explains some of the differences between column densities cal-
culated in our work and those reported in Cernicharo et al. (1999) especially toward
W51e1 e2 , at 66 km/s. The 21,2−10,1 transition of ortho c-C3 H2 at 85.338 GHz
(excitation energy E/k of 4.1 K) is free of this problem because it shows only ab-
sorption profiles at velocities where the 11,0−10,1 spectrum is contaminated by the
emission from the background source.
Fossé et al. (2001) mapped in detail the region around the cyanopolyyene
peak (hereafter CP) in TCM-1 in both l-C3 H2 and c-C3 H2 . They compared
two positions separated by 40 ′′ (which corresponds to 0.02 pc for the adopted dis-
tance of 100 pc to TCM-1; (Cernicharo 1987) and derived the following Nc/Nl
ratios: at CP(0,0) 28±6, and at the edge of the TCM-1 filament (−40,0) 10±3.
These authors used the total column density (ortho + para) of c-C3 H2 , but if only
the ortho c-C3 H2 transition is used, assuming an ortho/para ratio of 3, a ratio
Nc/Nl = 7.5±2. Fossé et al. (2001) used the UMIST95 chemical network (Millar
et al. 1997) in a pure gas-phase scheme to model their observations. The general
trend is that Nc/Nl increases with increasing visual extinction. This agrees with
observations, since the ratio is lower in the diffuse medium than in the TCM-1
dark cloud. The cyclic-over-linear ratio can be separated into two regimes: “low”
(1 ≤ AV ≤ 2) and “high” (5 ≤ AV ≤ 10), AV = 3 being an intermediate case. In
the low extinction regime (diffuse and translucent clouds) Nc/Nl strongly depends
on the electron abundance. Why is the cyclic over linear ratio sensitive to ioniza-
tion fractionation? In the UMIST95 chemistry C+ can destroy both l-C3 H2 and
c-C3 H2 , whereas C reacts only with the linear isomer. The abundance ratio of
3–5 derived by Cernicharo et al. (1999) in the diffuse medium corresponds, as ex-
320 A. A.
pected, to a high electron abundance and moderate visual extinction. In the high
extinction regime, Nc/Nl is independent of the C+ /C ratio, while it is sensitive
to the (H+3 +C+)/O ratio. Indeed, in the UMIST95 database, the atomic oxygen
reacts only with l-C3 H2 . This reaction is negligible compared to other destruction
reactions (in particular, the reactions with C+ which affects both isomers) until
all carbon is locked into CO. It is thus inefficient in low visual extinction regions
where the abundances C and C+ remain high. On the other hand, it becomes the
principal destruction channel of l-C3 H2 at high visual extinction. The variation
of the cyclic-over-linear abundance ratio of C3 H2 can be understood as a conse-
quence of the competition between neutral-neutral and ion-neutral reactions in the
interstellar medium. We can say that low ratios are indicative of a high electron
abundance, while high ratios indicate a low electron abundance. In a more general
way, isomeric ratios can be used to probe physical conditions in different media
from the diffuse gas to dark clouds. The detection of l-C3 H2 in the diffuse gas is
important, because the cumulene carbon chains (H2 Cn ) could contribute through
their electronic transitions, to the DIBs. However, the results of Cernicharo et al.
(1999) indicate a low abundance of Cn H radicals in the diffuse interstellar medium.
These results were based on an old quantum chemical prediction of the electronic
dipole moment, 4.1 Debye (which did not take into account electron correlation
effects). However recent computations carried out by Wu et al. (2010) yield an
almost identical value of 4.162 Debye. Indeed column densities of l-C3 H2 needed
to explain the diffuse interstellar band should be some three orders of magnitude
higher than what is observed (Liszt et al. 2012).
5. Conclusion
Madden et al. (1989) calculated column densities of ortho c-C3 H2 toward sev-
eral strong Galactic background continuum sources. These data were used by Cer-
nicharo et al. (1999) to determine the ratio Nc/Nl in the spiral-arm clouds on the
line-of-sight toward: W51e1 e2 , W49N, W51D and Sgr B2. We have presented new
data for W51e1 e2 , W49N and have calculated revised N (c-C3 H2 ) values. The val-
ues of the cyclic-over-linear abundance ratio are low, 3–5, for diffuse and translu-
cent cloud (1 ≤ AV ≤ 3), and higher, 8–13, for spiral-arm clouds (5 ≤ AV ≤ 10)
(denser, darker, translucent objects). A high value is also derived for the edge of
the TCM-1 filament (Fossé et al. 2001). In the low-AV regime, the abundance
ratio is sensitive to the C+ /C ratio and the second regime to the (H+3 +C+)/O ra-
tio (Fossé et al. 2001). Isomeric ratios can be used to probe physical conditions
in different environments from the diffuse gas to dark clouds. Our results are in
agreement with the predictions of Cernicharo et al. (1999) that the difference in
Nc/Nl ratio between different environments is probably a consequence of the dis-
tinct mechanisms leading to the destruction of the cyclic and linear isomers or their
progenitors.
Vol. 62 321
REFERENCES
Cernicharo, J., and Guelin, M. 1987, A&A, 176, 299.
Cernicharo, J., Gottlieb, C.A., and Guelin, M., Killian, T.C., Paubert, G., Thaddeus, P., and Vrtilek,
J.M. 1991, ApJ, 368, L39.
Cernicharo, J., Cox, P., Fosse, D., and Gusten, R. 1999, A&A, 351, 341.
Cox, P., Gusten, R., and Henkel, C. 1988, A&A, 206, 108.
Fossé, D., Cernicharo, J., Gerin, M., and Cox, P. 2001, ApJ, 552, 168.
Gerin, M., Kaz´mierczak, M., and Jastrzebska, M. 2011, A&A, 525, A116.
Goicoechea, J.R., Rodriguez-Fernandes, N.J., and Cernicharno, J. 2004, ApJ, 600, 214.
Herbig, G.H. 1975, ApJ, 196, 129.
Herbig, G.H. 1995, Ann.Rev.A&A, 33, 19.
Liszt, H., Sonnentrucker, P., Cordirer, M., and Gein, M. 2012, ApJ, 753, 28.
Lucas, R., and Liszt, H. 2000, A&A, 358, 1069.
Madden, S.C., Irvine, W.M., and Matthews, H.E., and Friberg, P. 1989, AJ, 97, 1403.
Millar, T.J., Farguhar, P.R. A., and Willacy, K. 1997, A&AS, 121, 139.
Oka, T., Thoburn, J.M., McCall, B.J., Friedman, S.D., Hobbs, L.M., Sonnentrucker, P., Welty, D.E.,
and York, D.G. 2003, ApJ, 582, 823.
Penzias, A.A., and Burrus, C.A. 1973, Ann. Rev. Astron. Astrophys., 11, 51.
Pety, J., Teyssier, D., Fossé, D., Gerin, M., Roueff, E., Abergel, A., Habart, E., and Cernicharo, J.
2005, A&A, 435, 885.
Thaddeus, P., Vrtilek, J.M., and Gottlieb, C.A. 1985, ApJ, 299, L63.
Wu,Q., Cheng, Q., Yamaguchi, Y., Li, Q., and Schaefer, H.F. 2010, J. Chem. Phys., 132, 44308.
van Dishoeck, E.,F., and Black, J.H. 1986, ApJS, 62, 109.
Vrtilek, J.M., Gottlieb, C.A., Gottlieb, E.W., Killian, E.W., and Thaddeus, P. 1990, ApJ, 364, L53.


ACTA ASTRONOMICA
Vol. 62 (2012) pp. 313–321
A Comparison of c-C3H2 and l-C3H2 in the Spiral Arm Clouds
M. K u l c z a k - J a s t r z ę b s k a1 , D. L i s 2 and M. G e r i n3
1Astronomical Observatory of the Jagiellonian University,30-244 Orla 171, Kraków,
Poland
e-mail:kulczak@oa.uj.edu.pl
2California Institute of Technology, Cahill Center for Astronomy and Astrophysics
301-17, Pasadena, CA 91125, USA
e-mail:dcl@caltech.edu
3LERMA, UMR 8112 du CNRS, École Normale Supérieure and Observatoire de Paris,
24 Rue Lhomond, 75231 Paris cedex 05, France
e-mail:maryvonne.gerin@lra.ens.fr
Received August 29, 2012
ABSTRACT
Using the IRAM 30-m telescope, we observed molecular absorption lines from c-C3H2 pro-
duced in diffuse clouds toward the high-mass star forming regions W51 e1/e2 and W49N to deter-
mine the abundance ratio between the cyclic and linear isomers of C3H2 (Nc/Nl ). The abundance
ratio is found to be 3–5 in the sources where l-C3H2 was previously detected. A possible source of
uncertainty in the determination ofNc/Nl is related to the estimate ofN (c-C3H2 ). The main goal
of this paper is verification of this hypothesis.
Key words: Radio lines: ISM – ISM: molecules – ISM: abundances
1. Introduction
Carbon is the fourth most abundant element in the interstellar medium (ISM),
and also the most versatile for building molecules. Carbon participates in numer-
ous chemical reactions at any temperature, from the very cold dense cores, to warm
and hot gas. Of the over 150 molecules detected in interstellar and circumstel-
lar environments about 75% have at least one carbon atom, while one fourth are
hydrocarbons. Moreover the heaviest and most complex molecules are organic
molecules with carbon. This statistic does not take into accunt the Polycyclic Aro-
matic Hydrocarbons (PAHs), nor the Diffuse Infrared Band (DIB) carriers, which
are more likely large organic molecules (Herbig 1995). While CH, CH+ and CN
have been known in the ISM since the 1940, the detection of other molecules in
the diffuse gas has decreased afterwords. However recent, deep observations have
314 A. A.
revealed and confirmed the presence of diatomic, triatomic and even more com-
plex molecules in the diffuse gas. Recent studies have shownthat the C-rich dif-
fuse interstellar medium is more chemically active than previously thought, with
molecules as large as C3 (Goicoecheaet al. 2004, Okaet al. 2003) and c-C3H2
(Lucas and Liszt 2000, Gerinet al. 2011) (c-C3H2 is the most widespread inter-
stellar ring molecule). The abundances of polyatomic molecules, such as c-C3H2 ,
HCO+ , HCN and C2H are somewhat higher than can be explained by conven-
tional chemical models (van Dishoeck and Black 1986). Smallcarbon containing
molecules, with a total of about 1 to 3 carbon atoms have several interesting prop-
erties: they show a widespread spatial distribution in the int rstellar medium, they
likely participate in the formation of long carbon molecules and are involved in
photo-fragmentation process of larger species, such as PAHs. It is widely believed
that carbon chain or ring species might be candidates for theDiffuse Interstellar
Bands (Herbig 1975, 1995).
2. Observations
Absorption line studies toward strong background continuum sources are of
interest because they allow probing diffuse, low-density clouds. The determination
of the line opacity is independent of the excitation temperature if Tex ≪ Tc , where
Tc is the brightness temperature of the continuum source. Suchabsorption line
observations also allow measurements of column densities on or two orders of
magnitude lower than those obtained through emission line studies.
2.1. Cyclic and Linear Forms of C3H2
Cyclopropelidene c-C3H2 was the first cyclic molecule to be detected in the
ISM (Thaddeuset al.1985, Vrtileket al.1990) and subsequently found to be ubiq-
uitous throughout the Galaxy, even in the diffuse gas (Coxet al. 1988). Its linear
isomer propadienlidene, H2CCC, is the first stable member of the cumulene car-
bon chains, characterized by double bonds between the carbons and terminal non-
bonded electrons. Propadienlidene has also been observed in a number of sources,
albeit at a lower abundance. It was first detected by Cernicharo (1991) in the cold
dark cloud TMC1. The observed abundance ratio between cyclic and linear C3H2
in TCM1 is 70 (Cernicharoet al. 1991). In the IRC+10216 circumstellar enve-
lope this ratio has a value of at least 30, whereas for the diffuse gas values lower
typically by a factor of ten have been derived.
2.2. Spiral-Arm Clouds
Cernicharoet al. (1999) detected linear C3H2 in absorption from the spiral-
arm clouds seen on the lines of sight toward giant HII regionsn the Galactic plane
(W49N, W51, Sgr B2). We have also studied some of these sources (Table 1).
Physical conditions in the spiral-arm clouds are supposed to overlap with those
Vol. 62 315
we inferred for the diffuse and translucent gas occulting extragalactic continuum
sources. For the spiral-arm clouds13CO emission is relatively strong and we con-
clude that they are more akin to the denser, darker, translucent objects studied by
Turner (1998). So the spiral-arm clouds toward W49 are at least 3–4 times higher
in column density than the most opaque of the clouds studied by Lucas (2000).
T a b l e 1
Coordinates and distances of the target sources
Source name α (J2000) δ (J2000) D [kpc] vLSR [km/s]
W51e1/e2 19h23m43.s9 +14◦30′25.′′9 7.0 56.0
W49N 19h10m13.s2 +09◦06′12′′ 11.4 8.4
2.3. Data Analysis and Processing
The observations were performed with the IRAM 30-m telescope in 2006 Au-
gust and December. We used the A100 and B100 receivers simultaneously with
either the A230, B230 or the C150, D150 receivers. All receivers were operated in
the single sideband mode, with a rejection of the image sideband etter than a fac-
tor of 20. The observations were performed using the wobbling secondary reflector
to obtain flat and stable baselines and an accurate measurement of the 3 mm radio
continuum intensity. The wobbler throw was 240′′and the switching rate 0.5 Hz.
The weather conditions were average, with 5–10 mm of precipitable water vapor
in August 2006 and less than 3 mm in December 2006. The pointing was checked
on nearby planets and continuum sources, and was found accurate to within 5′′ .
Spectra were recorded with the flexible, high spectral resolution back-end VESPA,
tuned to the spectral resolution of 40 kHz and the spectral bandp ss of 120 MHz,
covering a velocity range of 400 km/s. We also used the 1 MHz filterbank to obtain
broad-band spectra. We searched for the 21,2−10,1 transition of ortho c-C3H2 at
85.338 GHz. The data processing was done using the IRAM GILDAS software
package (Pety 2005). The resulting IRAM 30-m spectra are shown in Figs. 1, 2
and 3.
The data were first calibrated to theT∗A scale using the chopper wheel method
(Penzias and Burrus 1973), and subsequently converted to the main beam bright-
ness temperature scale using efficiencies appropriate for 85 GHz: Feff = 0.95 and
Beff = 0.75. The continuum intensityTmb(c) was measured by fitting flat baselines
to the spectra. Line parameters of the detected absorption features: central opac-
ity τ , velocity v and line width∆V, were derived by fitting Gaussian absorption
316 A. A.
Fig. 1. Spectra of c-C3H2 toward W51 and W49N obtained with the IRAM 30-m telescope. The
velocity scale is given relative to the Local Standard of Rest (LSR). The spectra have been offset for
clarity
Fig. 2. Absorption spectra of c-C3H2 toward W51 (the emission lines were removed). The source
continuum temperature was about 0.95 K
Vol. 62 317
Fig. 3. Absorption spectra of c-C3H2 toward W49N (the emission lines were removed). The source
continuum temperature was about 1.95 K
T a b l e 2
Line Gaussian parameters of the c-C3H2 absorption components toward W51e1e2 and W49
Source name vLSR ∆V Tmb Tmb(c) τ
[km/s] [km/s] [K] [K]
W51e1e2 5.6 3.36±0.48 −0.13 1.16 0.12±0.04
45.6 1.06±0.14 −0.11 1.16 0.10±0.04
65.1 2.89±0.14 −0.43 1.16 0.46±0.06
W49N 15.2 1.54±0.14 −0.48 2.34 0.22±0.02
34.1 3.36±0.14 −0.24 2.34 0.11±0.02
39.5 2.38±0.14 −0.50 2.34 0.24±0.02
59.5 3.18±0.17 −0.35 2.34 0.16±0.02
62.9 2.54±0.13 −0.49 2.34 0.23±0.02
profiles to the spectra. The central opacity is defined as:
τ = − ln
(
1−
Tmb
J(Tex)−J(Tcmb)−Tmb(c)
)
(1)
whereTmb is the line temperature with respect to the continuum,J(T) is the black
body function(hνk )/(e
hν
kT −1) . The excitation temperature (Tex) of the transition is
318 A. A.
typically not much higher then 2.7 K (the cosmic background radiation temperature
Tcmb), since the densities of the regions where the lines formed ar so low that the
molecules are not collisionally excited. Under this assumption the optical depth of
absorption line will be given directly from line to continuum intensity ratio as:
τ = − ln
(
1+
Tmb
Tmb(c)
)
. (2)
The line parameters derived from the IRAM observation of thec-C3H2 transition
are listed in Table 2.
3. Results
We compute molecular column densities using the relation:
Ntot = Z(Tex) ·Nu ·
eEu/kTex
gu
= Z(Tex)
8πν3
c3
·
eEu/kTex
guAul ·ehν/kTex−1
Z
τ · dυ (3)
whereZ(Tex) is the partition function computed at the excitation temperatureTex,
Ntot andNu are the total column density and the column density in the upper state
of the transition, respectively,gu is the statistical weight of the upper level,Aul is
the Einstein spontaneous emission coefficient,ν is the line frequency and
R
τ · dυ
is the line opacity integrated over the line profiles. For a Gaussian line profile the
integrated line opacity is proportional to the product of the line full width at half
maximum∆V · τ and the central opacityτ . The factor is very close to unity, and
can be written as
√
π/4ln2≃ 1.065. The ortho c-C3H2 column densities were
derived from relation of Lucas and Liszt (2000):
N(c-C3H2) = 4.36·10
12
·1.065·∆V · τ [cm−2]. (4)
The results of calculation are shown in Table 3. We compare ouestimates of the
abundance ratio between the cyclic and linear isomer of C3H2 with those found by
Cernicharoet al. (1991).
4. Discussion
Cernicharoet al. (1999) compared their column densities of l-C3H2 with the
Maddenet al. (1989) measurements of the (11,0−10,1) ortho c-C3H2 rotational
transition at 18.343 GHz (excitation energyE/k of 0.9 K) toward high mass star
forming regions (W51e1e2 , W49N, W51D and Sgr B2). In W51e1 2 and W49N,
the 11,0−10,1 transition shows complex emission/absorption profiles where the
emission corresponds in velocity with the HII region itself, and absorption ap-
pears at velocities corresponding to intervening cold clouds. Toward W51e1e2 ,
a narrow absorption feature at 66 km/s is contaminated by a bro d emission fea-
ture (∆V = 12 km/s) of the same molecule at the source velocity. A similarly
Vol. 62 319
T a b l e 3
Results
Source name v∗LSR vLSR N(l-C3H2) N(c-C3H2)
∗ N(c-C3H2) Nc/N∗l Nc/Nl
[km/s] [km/s] [cm−2] [cm−2] [cm−2]
W51e1e2 6.0 5.6 < 4.0E11 1.7E12 1.92E12 > 4.25 > 4.80
45.5 45.6 < 1.0E11 0.5E12 0.5E12 > 5.0 > 5.0
66.3 65.12 4.7E11 1.6E12 6.17E12 3.4 13.1
W49N 15.8 15.2 4.0E11 3.0E12 1.60E12 7.5 4.00
34.1 34.1 < 4.5E11 3.5E12 1.69E12 > 7.8 > 3.84
39.1 39.5 < 2.6E11 3.5E12 2.61E12 > 12.3 > 10.0
60.0 59.6 < 2.9E11 2.5E12 2.38E12 > 8.6 > 8.20
63.0 62.9 < 2.2E11 2.8E12 2.71E12 > 12.7 > 12.3
∗Cernicharo 1999
complicated spectrum of c-C3H2 at 18.343 GHz is observed between 0–20 km/s
toward W49N region. The line absorption profile contaminated by the emission
feature could be a source of error because the contaminationwould increase the
valueNc/Nl . This explains some of the differences between column densiti s cal-
culated in our work and those reported in Cernicharoet al.(1999) especially toward
W51e1e2 , at 66 km/s. The 21,2−10,1 transition of ortho c-C3H2 at 85.338 GHz
(excitation energyE/k of 4.1 K) is free of this problem because it shows only ab-
sorption profiles at velocities where the 11,0−10,1 spectrum is contaminated by the
emission from the background source.
Fosséet al. (2001) mapped in detail the region around the cyanopolyyene
peak (hereafter CP) in TCM-1 in both l-C3H2 and c-C3H2 . They compared
two positions separated by 40′′ (which corresponds to 0.02 pc for the adopted dis-
tance of 100 pc to TCM-1; (Cernicharo 1987) and derived the following Nc/Nl
ratios: at CP(0,0) 28±6, and at the edge of the TCM-1 filament(−40,0) 10±3.
These authors used the total column density (ortho + para) ofc-C3H2 , but if only
the ortho c-C3H2 transition is used, assuming an ortho/para ratio of 3, a ratio
Nc/Nl = 7.5±2. Fosséet al. (2001) used the UMIST95 chemical network (Millar
et al. 1997) in a pure gas-phase scheme to model their observations. The general
trend is thatNc/Nl increases with increasing visual extinction. This agrees with
observations, since the ratio is lower in the diffuse mediumthan in the TCM-1
dark cloud. The cyclic-over-linear ratio can be separated into two regimes: “low”
(1≤ AV ≤ 2) and “high” (5≤ AV ≤ 10), AV = 3 being an intermediate case. In
the low extinction regime (diffuse and translucent clouds)Nc/Nl strongly depends
on the electron abundance. Why is the cyclic over linear ratio sensitive to ioniza-
tion fractionation? In the UMIST95 chemistry C+ can destroy both l-C3H2 and
c-C3H2 , whereas C reacts only with the linear isomer. The abundanceratio of
3–5 derived by Cernicharoet al. (1999) in the diffuse medium corresponds, as ex-
320 A. A.
pected, to a high electron abundance and moderate visual extinction. In the high
extinction regime,Nc/Nl is independent of the C+ /C ratio, while it is sensitive
to the (H+3 +C
+)/O ratio. Indeed, in the UMIST95 database, the atomic oxygen
reacts only with l-C3H2 . This reaction is negligible compared to other destruction
reactions (in particular, the reactions with C+ which affects both isomers) until
all carbon is locked into CO. It is thus inefficient in low visual extinction regions
where the abundances C and C+ remain high. On the other hand, it becomes the
principal destruction channel of l-C3H2 at high visual extinction. The variation
of the cyclic-over-linear abundance ratio of C3H2 can be understood as a conse-
quence of the competition between neutral-neutral and ion-neutral reactions in the
interstellar medium. We can say that low ratios are indicative of a high electron
abundance, while high ratios indicate a low electron abundance. In a more general
way, isomeric ratios can be used to probe physical conditions n different media
from the diffuse gas to dark clouds. The detection of l-C3H2 in the diffuse gas is
important, because the cumulene carbon chains (H2Cn) could contribute through
their electronic transitions, to the DIBs. However, the results of Cernicharoet al.
(1999) indicate a low abundance of CnH radicals in the diffuse interstellar medium.
These results were based on an old quantum chemical prediction of the electronic
dipole moment, 4.1 Debye (which did not take into account electron correlation
effects). However recent computations carried out by Wuet al. (2010) yield an
almost identical value of 4.162 Debye. Indeed column densities of l-C3H2 needed
to explain the diffuse interstellar band should be some three orders of magnitude
higher than what is observed (Lisztet al.2012).
5. Conclusion
Maddenet al.(1989) calculated column densities of ortho c-C3H2 toward sev-
eral strong Galactic background continuum sources. These data were used by Cer-
nicharoet al. (1999) to determine the ratioNc/Nl in the spiral-arm clouds on the
line-of-sight toward: W51e1e2 , W49N, W51D and Sgr B2. We have presented new
data for W51e1e2 , W49N and have calculated revisedN(c-C3H2) values. The val-
ues of the cyclic-over-linear abundance ratio are low, 3–5,for diffuse and translu-
cent cloud (1≤ AV ≤ 3), and higher, 8–13, for spiral-arm clouds (5≤ AV ≤ 10)
(denser, darker, translucent objects). A high value is alsoderived for the edge of
the TCM-1 filament (Fosséet al. 2001). In the low-AV regime, the abundance
ratio is sensitive to the C+ /C ratio and the second regime to the(H+3 +C
+)/O ra-
tio (Fosséet al. 2001). Isomeric ratios can be used to probe physical conditions
in different environments from the diffuse gas to dark clouds. Our results are in
agreement with the predictions of Cernicharoet al. (1999) that the difference in
Nc/Nl ratio between different environments is probably a consequence of the dis-
tinct mechanisms leading to the destruction of the cyclic and linear isomers or their
progenitors.
Vol. 62 321
REFERENCES
Cernicharo, J., and Guelin, M. 1987,A&A, 176, 299.
Cernicharo, J., Gottlieb, C.A., and Guelin, M., Killian, T.C., Paubert, G., Thaddeus, P., and Vrtilek,
J.M. 1991,ApJ, 368, L39.
Cernicharo, J., Cox, P., Fosse, D., and Gusten, R. 1999,A&A 351, 341.
Cox, P., Gusten, R., and Henkel, C. 1988,A&A, 206, 108.
Fossé, D., Cernicharo, J., Gerin, M., and Cox, P. 2001,ApJ 552, 168.
Gerin, M., Kaźmierczak, M., and Jastrzebska, M. 2011,A&A, 525, A116.
Goicoechea, J.R., Rodriguez-Fernandes, N.J., and Cernicha o, J. 2004,ApJ, 600, 214.
Herbig, G.H. 1975,ApJ, 196, 129.
Herbig, G.H. 1995,Ann.Rev.A&A, 33, 19.
Liszt, H., Sonnentrucker, P., Cordirer, M., and Gein, M. 201, ApJ, 753, 28.
Lucas, R., and Liszt, H. 2000,A&A, 358, 1069.
Madden, S.C., Irvine, W.M., and Matthews, H.E., and Friberg, P. 1989,AJ, 97, 1403.
Millar, T.J., Farguhar, P.R. A., and Willacy, K. 1997,A&AS, 121, 139.
Oka, T., Thoburn, J.M., McCall, B.J., Friedman, S.D., Hobbs, L.M., Sonnentrucker, P., Welty, D.E.,
and York, D.G. 2003,ApJ, 582, 823.
Penzias, A.A., and Burrus, C.A. 1973,Ann. Rev. Astron. Astrophys., 11, 51.
Pety, J., Teyssier, D., Fossé, D., Gerin, M., Roueff, E., Abergel, A., Habart, E., and Cernicharo, J.
2005,A&A, 435, 885.
Thaddeus, P., Vrtilek, J.M., and Gottlieb, C.A. 1985,ApJ, 299, L63.
Wu,Q., Cheng, Q., Yamaguchi, Y., Li, Q., and Schaefer, H.F. 2010,J. Chem. Phys., 132, 44308.
van Dishoeck, E.,F., and Black, J.H. 1986,ApJS, 62, 109.
Vrtilek, J.M., Gottlieb, C.A., Gottlieb, E.W., Killian, E.W., and Thaddeus, P. 1990,ApJ, 364, L53.


https://authors.library.caltech.edu/101707/1/pap_62_3_6.pdf

A Comparison of c-C₃H₂ and l-C₃H₂ in the Spiral Arm Clouds

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main

Caltech Authors - Main