The Astrophysical Journal, 680: L121–L124, 2008 June 20
䉷 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
A SEARCH FOR PHOSPHINE IN CIRCUMSTELLAR ENVELOPES: PH3 IN IRC ⫹10216 AND CRL 2688?
E. D. Tenenbaum1 and L. M. Ziurys1,2
Received 2008 April 24; accepted 2008 May 9; published 2008 June 3
ABSTRACT
We present the results of a search for the JK p 10 r 0 0 transition of PH3 (phosphine) at 267 GHz toward
several circumstellar envelopes using the Arizona Radio Observatory 10 m Submillimeter Telescope (SMT). In
the carbon-rich shells of IRC ⫹10216 and CRL 2688, we have detected emission lines exactly at the PH3
frequency. Toward the oxygen-rich supergiant VY Canis Majoris, only an upper limit was obtained, while in the
evolved carbon-rich proto–planetary nebula CRL 618, the transition is contaminated by vibrationally excited
HC3N (n7 p 4). The line shape in IRC ⫹10216 appears to consist of two distinct components: a flat-topped
profile with a width of ∼28 km s⫺1, as is typical for this source, and a narrower feature approximately 4 km s⫺1
wide. The narrow component likely arises from the inner envelope (r ! 8R∗ ) where the gas has not reached the
terminal expansion velocity, or it is nonthermal emission. Based on the broader component, the abundance of
PH3 with respect to H2 is estimated to be 5 # 10⫺8 in a region with a radius of r ! 150R∗ . If the narrower
component is thermal, it implies a phosphine abundance of ∼5 # 10⫺7 close to the stellar photosphere (r !
8R∗). In CRL 2688, the PH 3 abundance is less constrained, with plausible values ranging from 3 # 10 ⫺8 to 4
# 10⫺7, assuming a spherical distribution. Phosphine appears to be present in large concentrations in the inner
envelope of C-rich AGB stars, and thus may function as a parent molecule for other phosphorus species.
Subject headings: astrochemistry — circumstellar matter — ISM: molecules — radio lines: stars —
stars: individual (CRL 2688, IRC ⫹10216)
PH3 is a symmetric top species, and therefore its rotational
energy levels have K-ladder structure. It is also a hydride, and
as a consequence, most of the favorable transitions of this
molecule lie in the submillimeter and infrared regions. There
are only two sets of rotational transitions accessible from the
ground at submillimeter wavelengths, the J p 1 r 0 and
J p 3 r 2 lines near 266.9 GHz and 800.5 GHz, respectively.
The J p 1 r 0 transition has only one K component, K p 0,
while the J p 3 r 2 transition has three such components,
K p 0, 1, and 2. Unlike ammonia, phosphine has too high of
an energy barrier to allow for inversion transitions. The
JK p 10 r 0 0 transition is thus the most favorable for a groundbased study of PH3. The molecule has a dipole moment of 0.57
D (Davies et al. 1971), and hyperfine interactions due to the
I p 1/2 spin of the 31P and H nuclei are negligible, with splittings of ∼0.1 MHz in the lowest rotational transition (Cazzoli
& Puzzarini 2006).
A number of searches for the JK p 10 r 0 0 transition of PH3
at 267 GHz have been carried out, all resulting in nondetections.
In one case, Turner et al. (1990) used the NRAO 12 m telescope
to search for the PH3 in the clouds Orion KL, Sgr B2, and
W51M, and in the envelope of IRC ⫹10216. More recently,
Agúndez et al. (2007) conducted a search toward IRC ⫹10216
using the IRAM 30 m; these authors reported an upper limit
to the PH3 column density of Ntot ! 3 # 1013 cm⫺2 .
Here we report a renewed search for the JK p 10 r 0 0 transition of PH3 toward the circumstellar shells of IRC ⫹10216,
CRL 2688, CRL 618, and VY CMa. These observations were
carried out with the Arizona Radio Observatory (ARO) Submillimeter Telescope (SMT), using a new sideband-separating
ALMA prototype receiver with record sensitivities. We detected
lines at the frequency of this transition in IRC ⫹10216 and
CRL 2688 that are likely attributable to PH3. In this Letter we
describe our results and their implications for phosphorus
chemistry in circumstellar gas.
1. INTRODUCTION
The carbon-rich AGB star IRC ⫹10216 has the most chemically complex circumstellar envelope known to date. In this
shell, over 50 molecular species have been detected, including
three simple saturated hydride species: ammonia, silane, and
methane (Monnier et al. 2000; Hall & Ridgway 1978). Some
of these hydrides have also been detected in circumstellar gas
around other evolved stars. For example, toward the envelope
of the oxygen-rich supergiant VY Canis Majoris (VY CMa)
and the carbon-rich proto–planetary nebula CRL 2688, ammonia has been observed (Monnier et al. 2000; Nguyen-QRieu et al. 1986). These detections prompt the question: what
other small hydrides exist in material around evolved stars?
Given the recent detections of new circumstellar phosphorusbearing molecules, HCP, PN, CCP, and PO (Halfen et al. 2008;
Milam et al. 2008; Agúndez et al. 2007; Tenenbaum et al.
2007), perhaps it is realistic to expect that PH3 will also be
observable in objects such as IRC ⫹10216 and VY CMa.
The chemistry of PH3 is not well understood in circumstellar
gas. Ion-molecule reactions do not appear to have favorable
routes leading to this species (e.g., Millar 1991; Turner et al.
1990), suggesting it would not form in cold, quiescent clouds.
In the stellar case, however, some models propose that PH3 is
produced at LTE in the inner circumstellar envelope and should
be a dominant parent species for phosphorus chemistry (Willacy & Millar 1997; MacKay & Charnley 2001). Other calculations indicate that it has negligible abundance in the inner
shell and that the main molecular carrier of phosphorus in this
region is HCP (Agúndez et al. 2007). Circumstellar phosphine
may even be destroyed by neutral-neutral reactions involving
atomic carbon or H2S, as shown in recent laboratory and computational studies (Guo et al. 2007; Viana & Pimentel 2007).
1
Departments of Chemistry and Astronomy, Steward Observatory, Laplace
Center for Astrobiology, University of Arizona, 933 North Cherry Avenue,
Tucson, AZ 85721; [email protected], [email protected].
2
Arizona Radio Observatory, University of Arizona, Tucson, AZ 85721.
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Vol. 680
TABLE 1
Observations of the PH3: JK p 10 r 00 Transition toward
Circumstellar Envelopes
Source
IRC ⫹10216:
Narrow comp.
Broad comp.
CRL 2688
VY CMa
CRL 618
T∗A
(K)
VLSR
(km s⫺1)
DV1/2
(km s⫺1)
∫ T∗A dV
(K km s⫺1)
0.007 Ⳳ 0.001
0.008 Ⳳ 0.001
0.003 Ⳳ 0.001
≤0.003
⫺27.0
⫺26.3
⫺35.6
…
…
4Ⳳ1
28 Ⳳ 1
36 Ⳳ 1
…
…
0.04 Ⳳ 0.01
0.21 Ⳳ 0.02
0.11 Ⳳ 0.04
…
…
a
Note.—SMT beam size at 267 GHz is 28⬙.
a
Contaminated by HC3N.
and additional observations were done toward IRC ⫹10216 at
257.753 GHz (HC3N: n7 p 4, [l 5 , l 6 , l 7 ] p [0, 0, 0], J p
28 r 27) to investigate the presence of rotational emission from
vibrationally excited HC3N.
3. RESULTS
Fig. 1.—Spectra measured toward IRC ⫹10216, CRL 2688, and CRL 618
at the frequency of the JK p 10 r 00 transition of PH3, using the ARO SMT.
In IRC ⫹1026 (top) and CRL 2688 (middle), there is emission exactly at the
frequency of PH3, while in CRL 618 (bottom), this region is contaminated by
the J p 29 r 28 transition of HC3N in its n7 p 4, [l5 , l6 , l7 ] p [0, 0, 0]
state. Spectral resolution is 1 MHz (1.1 km s⫺1).
2. OBSERVATIONS
The data were collected between 2007 December and 2008
March with the ARO 10 m SMT on Mount Graham, Arizona,
using a dual-polarization, sideband-separating ALMA band 6
(210–280 GHz) prototype mixer. The temperature scale at the
SMT is given in units of TA∗ , where the radiation temperature
is defined as TR p TA∗/hb , and hb is the main beam efficiency.
Data were taken in the LSB (nIF p 6 GHz), and rejection of
the image sideband was 12–18 dB. The system temperatures
were typically Tsys ∼180–450 K, single sideband. The back end
used was a 2048 channel filter bank with 1 MHz resolution
configured in parallel mode (2 # 1024 channels). Observing
was conducted in beam-switching mode with a Ⳳ2⬘ subreflector throw. Local oscillator shifts were done to establish any
image contamination. At the frequencies observed, hb ∼ 0.72,
determined from spectral scans of Mars, and vb ∼ 28 . The
B1950.0 source positions used were a p 09h45 m14.8s, d p
13⬚30⬘40⬙ (IRC ⫹10216); a p 21h00m20.0s, d p 36⬚29⬘44⬙
(CRL 2688); a p 04h39m34.0s, d p 36⬚01⬘16⬙ (CRL 618);
and 07h20m54.7 s, d p ⫺25⬚40⬘12⬙ (VY CMa). Pointing corrections and focus values were obtained approximately every
2 hours from continuum observations of planets, and when no
suitable planets were available, line pointing was done on the
J p 3 r 2 line of HCO⫹ in NGC 7027. Spectra of the PH3
transition were obtained at 266.945 GHz (PH3: JK p 10 r 0 0),
The results of the search are shown in Figure 1. At the
phosphine transition frequency, 15 and 3 mK lines are observed
toward IRC ⫹10216 and CRL 2688, respectively. In CRL 618,
a 15 mK feature is present at the exact frequency of PH3, but
this line likely arises from a rotational transition of vibrationally
excited HC3N. Unlike the carbon-rich stars, the oxygen-rich
envelope of VY CMa showed no emission to within a 3 j noise
level of 5 mK (TA∗). The line parameters and upper limit intensities are listed in Table 1.
SO2 , CH3CH2CN, and HC3N all have rotational transitions
that closely coincide with the JK p 10 r 0 0 transition of PH3 .
Contamination by the JKa,Kc p 30 9,21 r 318,24 transition of SO2
or the JKa,Kc p 15 4,12 r 15 2,13 transition of CH3 CH2CN is easily
ruled out because neither of these species have been detected
in a carbon-rich circumstellar envelope; furthermore, these transitions have weak intensities. The J p 29 r 28 transition of
HC3N in its n7 p 4, [l 5 , l 6 , l 7 ] p [0, 0, 0] state lies at 266.943
GHz, 2 MHz from the PH3 line. While emission from the
ground state of HC3N is observed in IRC ⫹10216 and CRL
2688 (Cernicharo et al. 2000; Jewell & Snyder 1984), emission
from vibrationally excited states of this molecule is not prevalent in these objects, other than for n7 p 1. In contrast, in
CRL 618, emission from vibrationally excited HC3N dominates
the millimeter and submillimeter spectrum (Pardo et al. 2007).
In addition to the contaminating feature, four more lines of
vibrationally excited HC3N are seen in the spectrum of CRL
618, shown in Figure 1. None of these four HC3N features are
apparent in the corresponding spectrum of IRC ⫹10216.
In order to absolutely verify that the line at the phosphine
frequency is not due to HC3N, we also observed the J p
28 r 27 transition of this species in its n7 p 4, [l 5 , l 6 , l 7 ] p
[0, 0, 0] state at 257.753 GHz in IRC ⫹10216. At this frequency,
no emission was detected to a noise level of 7 mK (3 j), as
shown in Figure 2. In the IRAM 30 m survey of CRL 618 by
Pardo et al. (2007), the J p 28 r 27 and J p 29 r 28 transitions are equally intense (TA∗ ∼ 100 mK). If the observed 15 mK
line at 266.945 GHz in IRC ⫹10216 is from HC3N, then a line
of approximately equal intensity would be observed at 257.753
GHz. Therefore, either the emission observed in IRC ⫹10216
is due to PH3, or it is caused by a molecule whose laboratory
rotational spectrum has not yet been published.
As in the case of IRC ⫹10216, no additional vibrationally
excited HC3N lines appear in the spectrum of CRL 2688 (Fig. 1).
These two sources have been shown to have similar phosphorus
No. 2, 2008
SEARCH FOR PHOSPHINE IN CIRCUMSTELLAR ENVELOPES
L123
Fig. 2.—Spectrum of the HC3N n7 p 4, [l5 , l6 , l7 ] p [0, 0, 0], J p
28 r 27 transition at 257.753 GHz, observed toward IRC ⫹10216 using the
SMT. The lack of emission at this frequency rules out HC3N as the source of
the feature at the PH3 10 r 00 transition at 266.945 GHz. Spectral resolution
is 1 MHz (1.2 km s⫺1).
chemistries, with PN and HCP observed in both of their shells
at similar intensity levels (Milam et al. 2008). Thus, if the
observed line arises from PH3 in IRC ⫹10216, it will likely
account for the emission feature in CRL 2688.
Fig. 3.—Enlarged view of the IRC ⫹10216 spectrum shown in the top panel
of Fig. 1. The black line is the observed emission, which appears to consist
of a central narrow peak superimposed over a wider flat-topped feature. The
gray line is the fit assuming a two-component model (see text). The upper
right inset shows the model abundance distribution: the dashed and dotted
lines represent the respective narrow and broad components.
4. DISCUSSION
4.1. Interpretation of PH3 Line Profiles
The proposed phosphine transition observed toward IRC
⫹10216 has an unusual line profile. Displayed more closely in
Figure 3, the line consists of a narrow component (width ∼ 4
km s⫺1), superimposed over a flat-topped, broad feature with
DV1/2 ∼ 28 km s⫺1. There are a number of ways to interpret this
profile. One possibility is that the proposed PH3 line is optically
thick and unresolved, giving rise to a parabolic-shaped feature.
Although the line appears to be more rectangular with a central
spike, this emission could be parabolic given the signal-to-noise
level. As discussed in the next section, an optically thick profile
is difficult to generate for trace species such as PH3 . Another
possibility is maser activity, where radiative pumping involving
vibrational states of phosphine produces the sharp emission
spike. This case is unlikely because SiS and HCN maser emission
in IRC ⫹10216 appears either in transitions of vibrationally
excited states, or as displaced peaks on top of thermal ground
state lines caused by pumping at point locations near the dust
formation zone (Fronfrı́a Expósito et al. 2006; Lucas & Cernicharo 1989). A third scenario is that the line is composed of
box-shaped PH3 emission topped with the narrow 7 mK line
arising from the n 7 p 4, [l 5 , l 6 , l 7 ] p [0, 0, 0], J p 29 r 28
transition of HC3N. In IRC ⫹10216, however, the four transitions
of vibrationally excited HC3N (n 7 p 1) observed in the IRAM
survey all appeared to be thermal and box-shaped with line
widths of ∼28 km s⫺1 (Cernicharo et al. 2000).
A final possibility is that the PH3 emission comes from two
different components in the envelope. In this scenario, the narrow central peak arises from the inner region of the envelope
where the gas has not yet achieved the terminal outflow velocity, resulting in the 4 km s⫺1 line width. According to the
velocity profile given by Keady et al. (1988), this inner gasacceleration region occurs at radii between 1R∗ and 10R∗ . The
wider flat-topped feature must arise from beyond the acceleration zone, giving the full line width of 28 km s⫺1. It has the
typical shape of an unresolved, optically thin molecular source.
In CRL 2688, the PH3 emission has a weaker intensity and
the signal-to-noise level does not reveal any certain profile. To
a first approximation, the line appears to be flat topped, and
can be interpreted as unresolved, optically thin emission.
4.2. Abundance and Distribution
To investigate the PH3 line shape in IRC ⫹10216, we conducted radiative transfer calculations using the circumstellar envelope code of Bieging & Tafalla (1993). The respective temperature and gas density profiles in the model are T(r) p
˙
T∗ (r/R∗ )⫺0.7 and r(r) p M/4p
vexp r 2, where T∗ is the effective
stellar temperature of 2320 K, R∗ is the stellar radius of 6.5 #
1013 cm, Ṁ is the mass-loss rate of 3 # 10⫺5 M, yr⫺1 (Agúndez
& Cernicharo 2006), and vexp is the expansion velocity. The
adopted distance to IRC ⫹10216 is 150 pc. A radiation field
due to dust is also included, approximated as a 600 K blackbody
at a radius of 7.5 # 1014 cm (Bieging & Tafalla 1993). The
calculations assumed a spherical distribution, initiated at a specified radius, rinner . The PH3 abundance relative to H2 is given by
2
the function f (r) p f0 e⫺(r/router ) , where f0 is the abundance at
rinner and router is the radius where the initial abundance decreases
by a factor of 1/e. Only K p 0 levels from ground state orthoPH3 were considered in the calculation. The collisional excitation
rates used were those of ortho-NH3 with para-H2 calculated by
Danby et al. (1988). Given the exclusion of para-PH3 and K 1
0 levels, and a characteristic rotational temperature of 100 K,
the fractional abundances from the model must be multiplied by
a factor of 5 to give the total abundance. In the modeling, we
limited the PH3 abundance to less than 5.6 # 10⫺7, which is
the maximum value, assuming the solar phosphorus abundance
(Grevesse & Sauval 1998).
From our calculations, we are able to rule out optically thick,
unresolved PH3 emission in IRC ⫹10216. Using router values
ranging from 1 # 1015 cm (15R∗ , 0.5⬙) to 3 # 1016 cm
(460R∗ , 13⬙), we were unable to produce a parabolic line with
an intensity matching the observed line. Even with the maximum allowable PH3 abundance, all the profiles generated by
the modeling were flat-topped.
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In the two-component scheme, we could reproduce the observed line profile. The best-fit model spectrum is shown in
Figure 3, overlaid on the observed spectrum, along with the
calculated abundance distribution, displayed in the upper right
inset. The inner component was generated using f0 p 1.1 #
10⫺7, rinner p 7 # 10 13 cm (∼R∗), router p 5 # 10 14 cm (8R∗ ,
0.2⬙), and vexp p 3 km s⫺1. The outer component was best modeled by f0 p 1.5 # 10⫺8, rinner p 6.5 # 10 14 cm (10R∗ , 0.3⬙),
router p 1 # 10 16 cm (150R∗ , 4⬙), and vexp p 14.5 km s⫺1. Accounting for para-PH3 and the K 1 0 levels, the average abundances of PH3 over the region rinner ! r ! router are 5 # 10⫺7 and
5 # 10⫺8, for the narrow and broad components, respectively.
The PH3 column densities over the same region are 2 # 1018
cm⫺2 and 6 # 1015 cm⫺2 for the two components. PH3 appears
to be formed in high concentration close to the stellar photosphere, accounting for ∼90% of the available phosphorus. As it
flows outward, it is depleted, either through condensation or
chemical reactions. Considering the model assumptions and uncertainty in the telescope calibration scale, we estimate our abundance values to be accurate to within a factor of 3. Observation
of this transition with a larger telescope beam will help to confirm
and further constrain this distribution scheme.
Phosphine emission in CRL 2688 was also modeled, assuming
a spherical flow, using the parameters R∗ p 9 # 1012 cm, T∗ p
˙ p 1.7 # 10⫺4 M yr⫺1, and dis6500 K, vexp p 19 km s⫺1, M
,
tance p 1000 pc (Skinner et al. 1997; Truong-Bach et al. 1990).
The abundance and distribution could not be well constrained
because of the large distance to the source and the lack of multiple
transitions. Setting rinner to 9 # 1013 cm (10R∗), we were able
to reproduce the line profile using parameters ranging from
f0 p 8 # 10⫺9 and router p 1 # 1017 cm (7⬙) to f0 p 1 # 10⫺7
and router p 8 # 1015 cm (0.5⬙), corresponding to respective
average PH3 abundances of 3 # 10⫺8 and 4 # 10⫺7. We note
that these values represent rough estimates since the actual outflow of CRL 2688 is far more complex than this simple model
(cf. Nguyen-Q-Rieu et al. 1986).
Vol. 680
culations, Agúndez et al. (2007) propose a two-component distribution for HCP in IRC ⫹10216 that is similar to that described
here for PH3 . According to our study, almost all of the phosphorus takes the form of gaseous PH3 close to the photosphere
(∼8R∗). A fair amount is present out to ∼150R* as well, with
f ∼ 10⫺8. These findings suggest that there may be flaws in the
thermal equilibrium calculations, possibly stemming from inaccurate thermochemical reference data derived from poor approximations and outdated experiments. Another cause for these
discrepancies may be photospheric shocks, which alter the LTE
concentrations (Willacy & Cherchneff 1998).
The more radially extended components seen both in IRC
⫹10216 and CRL 2688 may be the remnants of the photospheric
phosphine reservoir. Another option is that PH3 forms via gasgrain reactions involving condensed-phase phosphorus and H2 .
This type of scheme has been suggested as the formation mechanism of other saturated circumstellar species such as silane and
ammonia (Monnier et al. 2000; Willacy & Cherchneff 1998).
Near 150R* , however, PH3 in IRC ⫹10216 is then converted to
other molecules, or absorbed onto grains as the gas temperature
decreases. In CRL 2688, the molecule is more extended (900R*
! router ! 11,000R*), and may be longer lived. Shocks associated
with the second, more violent phase of mass loss may be influencing the PH3 abundance in this object (Cox et al. 2000). CRL
2688 is more luminous than IRC ⫹10216, but photochemical
effects are still thought to be negligible due to the high dust
opacity in this object (Cox et al. 1997), although such photodestruction could be occurring in CRL 618.
This speculation is based on a one-line detection. Clearly
additional transitions of PH3 need to be observed in order to
confine the abundance and radial distribution. The J p 3 r
2 transitions near 800 GHz, and the n2 and n4 vibrational fundamentals at 10 mm, may be options for ground-based telescopes. The J p 2 r 1 lines and the THz transitions are excellent targets for the upcoming Herschel observations. This
Letter should inspire such measurements, which will help elucidate the role of PH3 in LTE circumstellar chemistry.
4.3. Circumstellar Phosphorus Chemistry
The most recent calculations of thermodynamic equilibrium
chemistry in carbon-rich circumstellar environments predict HCP
to be the dominant species near the photosphere; calculated abundances of PH3 are insignificant, at less than 10⫺12 (Agúndez et
al. 2007; Milam et al. 2008). Based on these equilibrium cal-
This research is supported by NSF grant AST 06-07803 and
the NASA Astrobiology Institute under cooperative agreement
CAN-02-OSS-02 issued through the Office of Space Science.
E. D. T. acknowledges financial support from the NSF Graduate
Research Fellowship Program.
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