A&A 467, L37-L40 (2007)
DOI: 10.1051/0004-6361:20077415
LETTER TO THE EDITOR
J. Cernicharo1 - M. Guélin2 - M. Agúndez1 - K. Kawaguchi3 - M. McCarthy4 - P. Thaddeus4
1 - Dpt. Molecular and Infrared Astrophysics, Instituto de Estructura
de la Materia, CSIC, Serrano 121, 28006, Spain
2 -
IRAM, Domaine Universitaire, 300 rue de la piscine,
38400 St Martin d'Hères, France
3 -
Department of Chemistry, Faculty of Science,
Okayama University, Tsushimanaka, Okayama 700-8530, Japan
4 -
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA,
USA
Received 6 March 2007 / Accepted 30 March 2007
Abstract
Aims. Following the recent detection of C6H- in the laboratory and in space we have succeeded in studying the microwave spectrum of C4H-. We report here the first detection in space of this negative ion.
Methods. We have observed in the envelope of the carbon star IRC +10216 five lines corresponding to the J = 9-8, 11-10, 12-11, 14-13 and 15-14 rotational transitions of C4H-. The C4H- lines have a cusped shape, denoting that this ion is formed in the outer part of the envelope, like its neutral counterpart C4H.
Results. The abundance of C4H- in IRC+10216 is 1/6 of the abundance of C6H- and 1/4200 of that of C4H.
Conclusions. The detection of C4H-, after that of C6H-, confirms the theoretical prediction that C-chain anions are abundant in interstellar clouds and yields a first measurement of the electron radiative attachment rates.
Key words: stars: individual: IRC+10216 - stars: carbon - radio lines: stars - astrochemistry - line: identification - stars: AGB and post-AGB
The presence of negative ions in the interstellar medium was predicted many years ago on general grounds or on the basis of ion-molecule chemical models (see e.g. Blanksby et al. 2001; Petrie 1996; Herbst 1981; Millar et al. 2000; Dalgarno & McCray 1973). It was pointed out that a high electron affinity and a large number of vibrational states increase greatly the sticking coefficient of electrons, so that large, negatively charged carbon chains of the form CnH- may be abundant. In particular, Millar et al. (2000) predicted an abundance of C8H- as large as 1/4 of that of its neutral counterpart C8H in the outer envelope of the C-star IRC +10216, a source known to be particularly rich in C-chain molecules. For a long time, however, negative ions escaped detection because of the lack of accurate transition frequencies (see e.g. Morisawa et al. 2005). Recently, McCarthy et al. (2006) have succeeded in studying the microwave spectrum of C6H- in the laboratory and identified this species as the carrier of a series of lines observed by Kawaguchi et al. (1995) in the C-rich circumstellar envelope IRC +10216. A few years before, Aoki (2000) already proposed C6H- as a possible carrier of these lines on the basis of ab initio calculations. McCarthy et al.'s (2006) identification of C6H-in IRC +10216 and their subsequent detection of this species in the cold dark cloud TMC-1 demonstrate that negative ions can indeed be abundant in the interstellar medium. The abundance of C6H- derived in these two sources is as high as a few percent of that of C6H, considerably larger than the abundance of cations relative to their neutral counterparts, e.g. HC3NH+ relative to HC3N. Pursuing their laboratory work, McCarthy and colleagues have measured the rotational spectra of two more polyacetylenic anions, C8H- and C4H-(Gupta et al. 2007). In this Letter, we report the astronomical detection of the latter species in IRC+10216.
The 2 mm spectral line survey of
IRC +10216 (Cernicharo et al. 2000) is not sensitive enough to show lines
with intensities below 10 mK. In order to remove any doubt on the identification of
C4H- and to further constrain the partition function of the
rotational levels, we returned to the 30-m telescope to search for
two more C4H- transitions: the J = 14-13 transition at
130.3 GHz and the J = 15-14 transition at 139.6 GHz. The
observations were carried out in January 2007 with the 30-m
telescope. We observed simultaneously both C4H- transitions,
using the C and D 2-mm receivers. The backends consisted of two
filterbanks with
MHz-wide channels. The observing
mode, where we wobbled the secondary mirror by
at a rate of 0.5 Hz, and the dry weather conditions (sky
opacity at 225 GHz was below 0.1) ensured flat baselines
and low system noise temperatures (
K). The total ON+OFF integration time was 8 hours per line
yielding an rms noise
of
3 mK per 1 MHz channel.
In the 3 mm line survey of IRC+10216 we have also observed 12 lines of C6H-. The parameters for these lines are given in Table 1.
Table 1:
Number in parentheses are 1
uncertainties
in units of the last digits.
Observed C4H- and C6H- line parameters
![]() |
Figure 1:
J = 9-8, 11-10, 12-11, 14-13 and 15-14 lines
of C4H- toward IRC+10216. The spectral resolution is 1
MHz and the frequency scale (rest frequency) is relative to a
systemic velocity of -26.5 km s-1. The positions of the
C4H- lines are indicated by downward arrows. The lines of
HCCC13CCN correspond to its J = 42-41,
and 49-48 transitions. The corresponding transitions
of HCC13CCCN could be present in the same panels. However,
within the S/N of the data these lines are not detected.
Spectral intensities are in the antenna temperature scale,
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Table 1 gives the parameters for the observed lines of C4H- and C6H-. The C4H- lines show the characteristic cusped shape and sharp edges that are common to all optically thin lines arising from the outer envelope of IRC +10216. Those lines are centered at -26.5 km s-1 and have a full width of 29 km s-1, twice the expansion velocity of the envelope. The observed frequencies and widths of the J = 11-10, 14-13 and 15-14 lines agree well with these values. The J = 9-8 and 12-11 lines, although clearly visible, are partly blended with weaker lines of AlNC and HCCC13CCN. Thanks to the sharp edges and known widths, their rest frequencies can nevertheless be derived within 1 MHz. The good agreement of the observed and calculated line frequencies and the similarity of the line intensities, which are well accounted for by a single rotation temperature as will be seen below, confirm that we have detected C4H-.
As most reactive molecular species, neutral C4H is concentrated in IRC
+10216 inside an hollow shell of radius R = 14'' and thickness
(Guélin et al. 1993). The average rotation
temperature in the shell is
K and the total column
density (twice the radial column density) of C4H molecules in
the ground vibrational state
is N(C4H) =
cm-2(4/5 of all C4H, see Cernicharo et al. 2000).
Figure 2 shows the averaged spectrum of the
5 lines of C4H- in Fig. 1 superimposed on the corresponding
average spectrum of C4H
(divided by 100).
The strong similarity between the averaged line profiles of
both molecules indicates
that C4H and its anion are spatially coexistent.
Adopting for
C4H- a permanent dipole moment
D
(Botschwina, quoted by Blanksby et al. 2001), we derive
a rotational temperature
(C4H-) =
K, somewhat lower than that of C4H,
and a column density N(C4H-) =
cm-2 (see Fig. 3), which yields N(C4H-)/N(C4H) = 1/4200.
![]() |
Figure 2: Averaged spectrum of the J = 9-8, 11-10, 12-11, 14-13 and 15-14 lines of C4H- (histogram) and C4H (thick continuous line). All lines blended with those of C4H-have been fitted and removed. The C4H averaged spectrum has been scaled by a factor 100. |
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Moreover, once the weak AlNC and HCCC13CCN lines are
subtracted, the 5 observed C4H- line profiles, and the
corresponding ones
averaged for the neutral species, can be reproduced very well
with a radiative transfer MonteCarlo calculation assuming that C4H-and C4H are co-spatial (details will be given in a forthcoming paper).
This definitively shows that C4H- has a spatial
distribution not very different from that of C4H. These calculations
provide N(C4H-)/N(C4H)
1/4400, i.e., a very similar value to that
obtained from the rotational diagrams.
From all observed lines of C6H- (see Table 1)
we obtain N(C6H-) =
cm-2, similar to that reported by McCarthy et al. (2006), and
K (see Fig. 3).
A rotational temperature diagram
for the lines of neutral C6H (
)
observed in our 3 mm line survey
(Fig. 3) yields
K and N(C6H) =
cm-2,
after including the contribution
from the upper
spin ladder.
Thus, the ratio N(C6H-)/N(C6H) is 1/16
and N(C4H-)/N(C6H-) = 1/6.
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Figure 3:
Rotational diagrams for C4H-,
C6H- and C6H
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Herbst (1981) predicted from theoretical considerations that heavy
negative ions may be abundant in UV-shielded interstellar and
circumstellar clouds. Negative ions A- form by radiative
attachment of electrons on a neutral species A, a 2-step process
involving the formation of a complex (A-)* and its
stabilisation by emission of a photon:
![]() |
(1) |
![]() |
(2) |
In the cold dense gas,
should be proportional to the
density of vibrational states of A- at an internal energy equal
to the electron affinity EA (Herbst 1981). Thus,
is
high for large molecules with a high EA. Among those, the
polyacetylenic chain families C2nH, which have exceptionally
large electron affinities (
eV, Blanksby et al. 2001) and
whose smallest members had just been observed in IRC +10216
(Guélin et al. 1978) and in the dark cloud TMC-1 (Guélin et al. 1982), stuck out
as the most likely progenitors of large interstellar anions. It is
remarkable that with this simple reasoning, Herbst (1981)
predicted 26 years ago that C4H- and, more generally,
C2nH- (
)
may be abundant in interstellar and
circumstellar clouds ([C2nH-]/[C2nH] = 0.1-0.01).
The crucial, yet poorly known parameter controlling the abundance
of molecular ions is the radiative attachment rate coefficient
.
This coefficient is not measured, but only estimated
from statistical calculations (Terzieva & Herbst 2000; Petrie & Herbst 1997) or inferred from
laboratory three body reaction rate measurements (Herbst 1981).
According to Herbst and colleagues, the attachment rate for
polyacetylenic radicals with 6 or more C atoms may approach the
rate of collisions of heavy neutrals with electrons,
cm3s-1 (Terzieva & Herbst 2000; Petrie & Herbst 1997). Adopting a
rate that large, Millar et al. (2000) predicted that
[C8H-]/[C8H]
1/4 in IRC +10216. For smaller
species, such as C4H, the radiative stabilization of the
complex,
,
is slower than
its detachment,
,
and
may become much smaller than
(Terzieva & Herbst 2000).
Since the physical and chemical conditions in IRC+10216 and the chemical time scales are
relatively well known, we can try to derive from the measured [C4H-]/[C4H] and
[C6H-]/[C6H] ratios
the rates of radiative electron attachment,
,
for C4H and C6H.
Assuming, first, that steady
state is a valid approximation for the shell where CnH and
CnH- species are present, and neglecting photodetachment
(see below),
we find from Eq. (2):
![]() |
(3) |
With such low rates, however, steady state may not be fullfilled.
We have thus performed a time-dependent calculation using the
a chemical model of the circumstellar envelope of Agúndez & Cernicharo (2006)
to which we included reactions involving the negative ions
Cn- and CnH- (n = 4-9).
Photodetachment was also included with
a rate
exp(-2 Av) s-1.
In our models, this process is less efficient in destroying anions than
the reactions with neutral and cations by more than a factor of 10.
Reactions of negative charge exchange, e.g.,
A- + B
A + B-, are not included in our
models as they seem unimportant compared to the other processes.
The radiative electron attachment rates of species other than C4H
and C6H were set to
cm3 s-1, while
for C4H and C6H were varied to match
the observed [C4H-]/[C4H] and
[C6H-]/[C6H] ratios. Fig. 4 shows the
abundance profiles obtained from the model for CnH and
CnH- for n = 4, 6, 8. Although Fig. 4
shows that steady state is indeed a poor approximation, the values of
(C4H) and
(C6H) obtained from the
chemical model (
and
cm3 s-1) agree
well with those derived just above under
this assumption. Our values are somewhat
lower than the upper limits
(C4H) =
(T/300)-1/2 cm3 s-1 and
(C6H) =
(T/300)-1/2 cm3 s-1 calculated by
Herbst (2007, private communication).
Our model predicts that anions have a more extended
distribution than their neutral counterparts, which is related to the
increase of the electron abundance with radius. This behaviour
was also predicted by Millar et al. (2000).
![]() |
Figure 4: Abundance profiles of CnH and CnH- given by chemical modelling. Angular distance is given on the top axis for an assumed distance to the star of 150 pc. |
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The large [C6H-]/[C6H] ratio found in IRC +10216 and the
theoretical predictions of large
for increasing
molecular size, i.e. increasing [CnH-]/[CnH] for
increasing n, suggests that C8H- could be detected in IRC
+10216 given that C8H has been already detected (Cernicharo et al. (1996).
Our model indicates that [C8H-]/[C8H] is as high as 1/2, while
Millar et al. (2000) model predicted it to be 1/4. We have searched for
C8H- in our IRC +10216 data. Although the low rotational
constant of C8H- (B=583 MHz, Gupta et al. 2007) makes that
many rotational transitions are covered, the spectral line crowding
(which is a serious problem at the 2-3 mK level), and the high level
energies (which make it difficult to excite the 3-mm lines), leaves
only few transitions that could be reasonably observed in the 3-mm
domain. Figure 5 shows the deepest spectrum,
which was observed in the course of a search for HC4N
(Cernicharo et al. 2004). The rms noise on this spectrum is 0.7 mK per 1-MHz
channel. From this and other spectra we derive a 3-sigma upper
limit of [C8H-]/[C8H]
1/5, taking into account the
difference in permanent dipole moments (6.5 D for C8H versus
10.4 D for C8H-, Blanksby et al. 2001) and the effect of fine
structure splitting in C8H. Our limit is not low
enough to set significant constraints on the electron radiative
attachment rate on C8H.
Following the identification of C6H- by McCarthy et al. (2006) we have detected a second negative ion, C4H-, in the C-star envelope IRC +10216. The abundance decrement between the two ions, [C4H-]/[C6H-] = 1/6, is more than two orders of magnitude smaller than that between their neutral counterparts, [C4H]/[C6H] = 45. The two negative ions appear more abundant than their positive ion analogs which, so far, have not been identified.
The detection of C4H- and C6H- sustains the farsighted prediction of Herbst (1981) and co-workers that carbon-chain anions may be abundant in the interstellar medium. Other negative ions from the same family, such as CN-, C3N-, CCH- and C8H-, are likely to be detectable, although, probably, at lower intensity levels. Some of these species have already been studied in the laboratory and their millimeter rotational frequencies are accurately known, awaiting only of deeper astronomical searches. The observed [C4H-]/[C6H-] abundance ratio differs significantly from that estimated from statistical calculations of radiative electron attachment. This shows that surprising results are likely in this matter.
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Figure 5: Spectrum of IRC+10216 covering the J = 71-70 rotational transition of C8H-. The upward arrows indicate the position of the edges of the C8H- line. |
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Acknowledgements
We thank Spanish MEC for funding support trough grants AYA2003-2785, AYA2006-14876, ESP2004-665, and AP2003-4619 (MA), and by Spanish CAM under PRICIT project S-0505/ESP-0237 (ASTROCAM). We would like to thank Eric Herbst for useful comments and discussions.