The Astrophysical Journal Letters, 712:L93–L97, 2010 March 20
C 2010.
doi:10.1088/2041-8205/712/1/L93
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
EXOTIC METAL MOLECULES IN OXYGEN-RICH ENVELOPES: DETECTION OF AlOH (X1 Σ+ )
IN VY CANIS MAJORIS
E. D. Tenenbaum1 and L. M. Ziurys1,2
1
Department of Astronomy, Department of Chemistry, and Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA;
[email protected]
2 Arizona Radio Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA; [email protected]
Received 2009 December 21; accepted 2010 February 15; published 2010 March 4
ABSTRACT
A new interstellar molecule, AlOH, has been detected toward the envelope of VY Canis Majoris (VY CMa), an
oxygen-rich red supergiant. Three rotational transitions of AlOH were observed using the facilities of the Arizona
Radio Observatory (ARO). The J = 9 → 8 and J = 7 → 6 lines at 1 mm were measured with the ARO Submillimeter
Telescope, while the J = 5 → 4 transition at 2 mm was observed with the ARO 12 m antenna on Kitt Peak. The
AlOH spectra exhibit quite narrow line widths of 16–23 km s−1 , as found for NaCl in this source, indicating that
the emission arises from within the dust acceleration zone of the central circumstellar outflow. From a radiative
transfer analysis, the abundance of AlOH relative to H2 was found to be ∼1 × 10−7 for a source size of 0.26 or
22 R∗ . In contrast, AlCl was not detected with f 5 × 10−8 . AlOH is likely formed just beyond the photosphere via
thermodynamic equilibrium chemistry and then disappears due to dust condensation. The AlOH/AlO abundance
ratio found in VY CMa is ∼17. Therefore, AlOH appears to be the dominant gas-phase molecular carrier of
aluminum in this oxygen-rich shell. Local thermodynamic equilibrium calculations predict that the monohydroxides
should be the major carriers of Al, Ca, and Mg in O-rich envelopes, as opposed to the oxides or halides. The apparent
predominance of aluminum-bearing molecules in VY CMa may reflect proton addition processes in H-shell burning.
Key words: astrochemistry – circumstellar matter – ISM: molecules – radio lines: stars – stars: individual (VY
CMa) – supergiants
Online-only material: color figure
monohydroxide form is not the lowest energy isomer. Nitrogen,
carbon, sulfur, and phosphorus are more stable as HNO, HCO,
HSO, and HPO, respectively. Conversely, the elements silicon,
oxygen, magnesium, sodium, and potassium favor the XOH
isomer, but none of their hydroxides have been detected in
interstellar or circumstellar gas. In the case of SiOH, the
fundamental rotational transition at 33 GHz has recently been
recorded in the laboratory (McCarthy et al. 2008), but the lack of
higher frequency measurements makes an interstellar detection
problematic. MgOH, CaOH, and NaOH have been searched for
toward molecular clouds and the circumstellar envelope of IRC
+10216, but only upper limits have been reported (Turner 1991;
Sakamoto et al. 1998; Walmsley et al. 2002).
AlOH, another possible hydroxide candidate, was first identified in the laboratory in the gas phase by Pilgrim et al. (1993),
who measured the A–X and B–X electronic transitions using
photoionization spectroscopy. Shortly thereafter, Apponi et al.
(1993) recorded the millimeter-wave spectrum of this species,
providing highly accurate rest frequencies. Ziurys and collaborators then conducted an astronomical search for AlOH toward
IRC +10216 and Orion-KL within one year of the laboratory
study, but failed to detect the molecule. More recent computational work predicts a significant dipole moment of ∼1.0 D
in the ground electronic state of AlOH (Li et al. 2003). With
new SIS mixer technology pursued for the Atacama Large Millimeter Array (ALMA), receiver sensitivities have significantly
improved, warranting renewed searches for molecules that were
“missed” in the past, such as AlO (Tenenbaum & Ziurys 2009)
and PO (Tenenbaum et al. 2007). Both these species were detected toward VY CMa.
Here, we present the identification of aluminum hydroxide in
the envelope of VY CMa. Three rotational emission lines in the 1
1. INTRODUCTION
For many years, interstellar molecules containing a metal (in
the chemist’s sense) had only been detected in the envelopes
of carbon-rich evolved stars, in particular the asymptotic giant
branch star IRC +10216. In this object, four halide compounds,
including common table salt, have been identified (NaCl,
AlF, AlCl, and KCl), as well as a series of metal cyanide
and isocyanide species: MgNC, NaCN, MgCN, AlNC, and
tentatively KCN (Cernicharo & Guélin 1987; Ziurys et al. 1994,
1995, 2002; Kawaguchi et al. 1993; Turner et al. 1994; R. L.
Pulliam et al. 2010, in preparation). A few of these molecules
have also been discovered in the carbon-rich protoplanetary
nebulae CRL 2688 and CRL 618 (Highberger et al. 2001;
Highberger & Ziurys 2003). In the past two years, a new source
for metal-bearing molecules has been found, namely, the O-rich
shell of the red supergiant VY Canis Majoris (VY CMa). Toward
this object, the molecule AlO has recently been discovered
(Tenenbaum & Ziurys 2009), and NaCl has also been observed
(Milam et al. 2007). The oxygen-rich environment in VY CMa
appears to foster a novel refractory chemistry that previously
had been unexplored. In this Letter, we report on the detection
of another new molecule, AlOH, in the envelope of this object.
Monohydroxides, a class of molecules where an OH group
is bound to another atom, are rare in space. Water is the
only one of this type that has been detected in the interstellar
medium (ISM). The hydrogen peroxide radical, HO2 , another
possibility, has not been identified in the ISM thus far. This
species is predicted to form in ices irradiated with cosmic rays,
and may subsequently enter the gas phase during grain mantle
evaporation (Kaiser et al. 1999). One reason for the paucity
of hydroxides is that, for many of the abundant elements, the
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Vol. 712
Table 1
Line Parameters for AlOH in VY CMa
Transition
J=9→8
J=7→6
J=5→4
ηa
θb
( )
VLSR
(km s−1 )
TR
(mK)
ΔV1/2
(km s−1 )
b
Observed
TR dV
(K km s−1 )
0.78
0.78
0.72
27
34
40
21 ± 2.1
19 ± 2.7
19 ± 3.8
3.8 ± 0.7
2.0 ± 0.4
1.5 ± 0.5
21.2 ± 2.1
16.3 ± 2.7
23 ± 8
0.063 ± 0.006
0.036 ± 0.004
0.02 ± 0.005
Frequency
(MHz)
283,253.9
220,330.9
157,391.0
b
c
Model
TR dV
(K km s−1 )
0.085
0.035
0.013
Notes.
a η for SMT data and η for 12 m data (see the text).
b
c
b Reported error ranges are 2σ .
c Integrated line intensities predicted by the best-fit radiative transfer model (see the text).
and 2 mm atmospheric bands were observed using the Arizona
Radio Observatory (ARO) telescopes. In the following sections,
we discuss our observations, their analysis, and the implications
of this detection for circumstellar chemistry and supergiant
nucleosynthesis.
2. OBSERVATIONS
The observations were carried out between 2007 December
and 2009 June using the telescopes of the ARO: the 10 m
Submillimeter Telescope (SMT) on Mt. Graham, AZ, and the
12 m at Kitt Peak. Data at 1 mm were taken with the SMT using
a dual-polarization receiver featuring sideband-separating SIS
mixers developed for ALMA Band 6. Image rejection, built
into the mixer architecture, was typically 15 dB, and system
temperatures ranged from 200 K to 350 K in good weather
conditions. Filter banks with 1 MHz resolution, configured in
parallel mode (2 × 1024 channels), served as back ends. The
temperature scale at the SMT is measured as TA ∗ , derived by the
chopper-wheel method. The radiation temperature is defined as
TR = TA ∗ /ηb , where ηb is the main-beam efficiency.
The observations at 2 mm were conducted with the 12 m
telescope using a dual-polarization receiver. Typical image
rejection was 20 dB, determined by tuning the mixers, and
system temperatures ranged from 300 K to 600 K depending on
weather conditions. Two filter banks with 1 and 2 MHz spectral
resolution, respectively, were used as back ends, configured in
parallel mode (2 × 128 channels) for both receiver polarizations.
An autocorrelator set to 195 kHz resolution was also employed.
The temperature scale at the 12 m is in TR ∗ , which is the chopperwheel antenna temperature corrected for forward spillover
losses. Radiation temperature is defined as TR = TR ∗ /ηc , where
ηc is the corrected beam efficiency.
The data were taken in beam-switching mode with a ±2
subreflector throw toward VY CMa, using the coordinates
α = 07h 20m 54.7s δ = −25◦ 40 12 (B1950.0; Perryman et al.
1997). Pointing and focus were checked every 1–2 hr on
Saturn or Venus. Observing frequencies, beam sizes, and beam
efficiencies are listed in Table 1.
3. RESULTS
Three rotational transitions of AlOH were detected toward
the shell of VY CMa: two at 1 mm (J = 9 → 8 and J = 7
→ 6 lines at 283 and 220 GHz) and one at 2 mm (J = 5 → 4
transition near 157 GHz). AlOH is a closed-shell species with
a 1 Σ+ ground state. At lower J transitions, AlOH exhibits a
quadrupole hyperfine pattern due to the I = 5/2 spin of the 27 Al
nucleus. However, the hyperfine splitting is less than 1 MHz
at the observing frequencies, and thus unresolvable given the
line widths in VY CMa (∼30–40 km s−1 ). The observed spectra
are displayed in Figure 1. As the data show, the AlOH features
fall at VLSR of ∼20 km s−1 , typical of molecular emission from
VY CMa (Ziurys et al. 2009). The temperatures of the lines are
on the order of a few mK, with 1σ rms noise levels ranging
from 0.4 mK to 0.7 mK in the spectra. The J = 9 → 8 and
J = 7 → 6 lines are therefore detected at ∼5σ levels, and the
J = 5 → 4 line is detected at a 3σ level. Integration times for
the J = 9 → 8, J = 7 → 6, and J = 5 → 4 lines are 37 hr,
90 hr, and 127 hr, respectively. The AlOH features also exhibit
rather narrow line widths of 16–23 km s−1 (FWHM), as found
for NaCl in this source (see the inset in Figure 1). More common
circumstellar species such as CO, HCN, and SO2 have ΔV1/2
near 30–60 km s−1 in VY CMa (Ziurys et al. 2007). We also
searched for the J = 8 → 7 transition of AlOH at 251.8 GHz,
but were unable to detect the line because this transition is
completely obscured by the NJ = 65 → 54 feature of SO, which
has TA ∗ ∼ 0.8 K.
Spectral line catalogs were checked for possible contaminating emission from other molecules but no coincident transitions
were found. However, a number of other molecular features lie
near the AlOH lines. Adjacent to the J = 7 → 6 transition of
AlOH is the J = 2 → 1 line of 13 CO (see Figure 1). In addition,
emission lines from the JKa,Kc = 160,16 → 151,15 transitions of
both 32 SO2 and 34 SO2 fall near the J = 9 → 8 feature.
4. DISCUSSION
4.1 Abundance and Distribution of AlOH in VY CMa
Millimeter wave emission lines from VY CMa exhibit
between one and three velocity components, depending on
the molecule and transition. A central component, at VLSR
∼20 km s−1 , marks a spherical outflow, while separate, distinct features at ∼42 km s−1 and −7 km s−1 arise from red- and
blue-shifted collimated jets (see Ziurys et al. 2007, 2009). Typical central flow line widths (full width zero power or FWZP) are
∼60 km s−1 (Ziurys et al. 2007), reflecting the terminal expansion velocity of 30 km s−1 . The observed AlOH lines only show
one velocity component at ∼20 km s−1 , indicating that emission
arises from just the spherical outflow. In addition, the features
have quite narrow line widths (16–23 km s−1 ), characteristic of
NaCl in this object, as mentioned. The narrow line widths in
these profiles indicate that AlOH arises from the inner part of
the outflow that has not reached terminal velocity, likely in the
dust acceleration zone.
To establish an abundance and source distribution, a non-LTE
radiative transfer model of circumstellar molecular emission
developed by Bieging & Tafalla (1993) was used to analyze
the AlOH spectra. The gas kinetic temperature and density
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EXOTIC METAL MOLECULES IN OXYGEN-RICH ENVELOPES
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0.13 (11 R∗ ). A value of rinner ∼ 5 × 10 cm was used, or ∼2
R∗ . To check for dependency on collisional excitation rates, we
also ran models using the CS–H2 cross sections (Turner et al.
1992) and found no significant difference in results.
Good agreement between the observed and predicted AlOH
features was found for an abundance relative to H2 of f0 ∼ 1
× 10−7 with a source size of ∼0.26 . The integrated intensities
of the calculated line profiles are given in Table 1. As the table
shows, the observed and predicted intensities agree to within
35%. We estimate that the derived abundance is accurate to
at worst, an order of magnitude, based on uncertainties of the
assumed source size, model approximations, and the observed
integrated intensities. An abundance near 10−7 indicates that
∼1% of the available aluminum is in the form of AlOH at about
10 R∗ from the star, assuming a cosmic elemental abundance
(Lodders 2003). Adopting a similar distribution for aluminum
monoxide and using the data from Tenenbaum & Ziurys (2009),
the abundance ratio of AlOH/AlO is ∼17. Hence, AlOH is the
dominant known gas-phase aluminum-bearing molecule in VY
CMa.
14
4.2 Metal Hydroxides: A New Class of Circumstellar
Molecules?
Figure 1. Spectra of the J = 9 → 8 and 7 → 6 transitions of AlOH (X1 Σ+ )
observed toward the circumstellar shell of VY Canis Majoris (VY CMa) using
the ARO SMT at 1 mm, as well as the J = 5 → 4 line of this molecule measured
at 2 mm with the ARO 12 m. The resolution for all data is 2 MHz, corresponding
to 2.1 km s−1 , 2.7 km s−1 , and 3.8 km s−1 for the N = 9 → 8, 7 → 6, and 5
→ 4 lines, respectively. Line widths are narrower than those of 13 CO and SO2
observed in this source, as shown in these spectra, but are similar to those of
NaCl (see the inset of NaCl: J = 19 → 18 transition at 247 GHz). Integration
times for the AlOH data, in order of lowest to highest frequency, are 127 hr,
90 hr, and 37 hr.
profiles adopted in the modeling are those for VY CMa from
Ziurys et al. (2009). A distance of 1.14 kpc (Choi et al. 2008)
and a spherical flow mass-loss rate of 2.6 × 10−4 M yr−1
(Humphreys et al. 2005; Monnier et al. 2000; adjusted for
updated distance) were assumed for this supergiant star. Due to
the lack of published AlOH collisional excitation rates, values
for the HCN–H2 system (Green & Thaddeus 1974) were used
as a substitute, and the first 30 rotational levels in the ground
vibrational state were considered. In the code, the abundance
of AlOH relative to H2 is described by the Gaussian function
r
2
f (r) = f0 e−( router ) . The value f0 is the abundance at rinner , where
the calculation is initiated, and router is the radius at which the
abundance decreases by a factor of 1/e with respect to f0 .
The observed narrow line widths for AlOH constrain the
distribution of this molecule around the star. FWHM and FWZP
line widths of the AlOH features indicate an expansion velocity
for this species between 10 km s−1 and 15 km s−1 . An expansion
velocity of 10 km s−1 is attained at r ∼ 0.13 , as deduced from
water maser observations over two epochs by Richards et al.
(1998). Hence, router was set to a value of 2.2 × 1015 cm, or
The distribution of AlOH within the dust acceleration zone
suggests formation by local thermodynamic equilibrium (LTE)
chemistry. At r ∼ 11 R∗ , the temperature is estimated to be
T ∼ 600 K, and the density n(H2 ) ∼ 108 cm−3 , nearly LTE
conditions. To examine the aluminum chemistry network, calculations of gas-phase LTE synthesis in an O-rich environment
were carried out. The results are shown in Figure 2. As demonstrated by the modeling, AlOH is predicted to be the most abundant aluminum-bearing molecule in the region where Tkin =
1500 K to 700 K (∼2–8 R∗ ; Tenenbaum & Ziurys 2009). The
LTE model shows an AlOH abundance gradually rising from
2 × 10−9 at 2500 K (1 R∗ ) to 2 × 10−6 by 1700 K (2 R∗ ).
The same calculation shows AlO achieving a peak abundance of
3 × 10−10 at 2500 K, essentially at the photosphere. AlCl, which
is abundant in IRC +10216, is also predicted to form close to
the photosphere, but only reaches a maximum abundance of
∼5 × 10−8 .
A comparison of observed and predicted abundances of
selected hydroxides, halides, and oxides in VY CMa is given
in Table 2. The observed abundances were derived using the
same radiative transfer model, collisional excitation rates, and
source size (θ s = 0.26 ) as described above for AlOH. When
compared with observations, the LTE model underestimates
the AlO abundance by a factor of 20 and overestimates the
AlOH abundance by about an order of magnitude. The observed
upper limit to AlCl in VY CMa is f 5 × 10−8 , comparable
to its maximum predicted concentration. Nevertheless, there
is a qualitative agreement between model predictions and the
observations for VY CMa; namely, AlOH is the most prominent
molecular carrier of gas-phase aluminum in this object.
If the hydroxide of a highly refractive element like aluminum
is present in the inner envelope of a circumstellar shell, then
other refractory monohydroxides may exist as well. Figure 2
additionally shows LTE chemistry predictions for the monohydroxides and monoxides of other metals in an O-rich envelope.
The monoxides all achieve their peak abundances at ∼1 R∗ ,
but they represent only minor sinks for calcium, magnesium,
potassium, and sodium. (Potassium monoxide is not indicated
in the figure because its maximum predicted abundance of 10−15
is well below the range of the graph.) The hydroxides become
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Vol. 712
Table 2
Abundances of Al, Na, and K-bearing Molecules in VY CMa
Species
AlOH
NaOH
KOH
AlCl
NaCl
KCl
AlO
Observed Abundancea
10−7
1×
3 × 10−9
1 × 10−9
5 × 10−8
4 × 10−9
4 × 10−10
6 × 10−9
LTE-predicted Abundance
2 × 10−6
1 × 10−7
8 × 10−9
4 × 10−8
1 × 10−7
6 × 10−8
3 × 10−10
Note. a Based on ARO observations of multiple transitions of each
molecule.
prevalent as the gas cools and expands, reaching their peak
abundances in the 3–8 R∗ region. At lower temperatures, the
calculations predict CaOH and MgOH to be the main carriers
of their respective metals, while the atomic forms dominate at
higher temperatures.
To test the model, we have also carried out searches toward
VY CMa for both NaOH and KOH, whose rest frequencies
are well known (Pearson & Trueblood 1973a, 1973b). Neither
species was detected. As shown in Table 2, the abundance limits
are f ∼1–3 × 10−9 , almost 2 orders of magnitude less abundant
than AlOH. However, sodium and potassium remain elusive
metals in the envelope of this supergiant star. The only sodiumcontaining species seen thus far is NaCl, which has an abundance
of f ∼ 4 × 10−9 (see Milam et al. 2007). Furthermore, our data
indicate that KCl is not present with an upper limit of f 4
× 10−10 . It is notable that CaOH has been observed in the
atmospheres of late M-dwarf stars via their visible absorption
spectra, proving that LTE formation of calcium hydroxide is a
reality (Pesch 1972). Clearly, more sensitive searches are needed
for CaOH and MgOH.
One fact does emerge from a comparison of observed and
calculated abundances in Table 2. The LTE predictions in general overestimate the molecular abundances. The discrepancy
between observed and predicted abundances is most probably
due to condensation and shock effects, which are not included
in the calculations. (A theoretical study by Sharp & Huebner
1990 on LTE circumstellar chemistry does account for both gas
and solid phase phenomena, but it does not report results for
AlOH). Condensation for aluminum-bearing species is likely to
be substantial in the inner envelope due to the high refractivity
of this metal. In an O-rich circumstellar outflow, aluminum is
one of the first elements to condense, forming Al2 O3 at 1700 K
(Lodders & Fegley 1999; Gail & Sedlmayr 1998), although it
is unclear how AlOH condenses into grains. Furthermore, H2 O
and SiO maser emissions in VY CMa indicate that shocked gas
is present in the inner envelope (Menten et al. 2008; Humphreys
2007), and it has been suggested that such shocks are the cause
of the highly directional outflows of molecular gas (Ziurys et al.
2007). In addition to causing irregular gas ejecta, shocks have
the potential to disrupt LTE and condensation chemistry (Cherchneff 2006). The effect of shock waves on metal chemistry,
however, has yet to be investigated theoretically.
4.3 Implications for Metal Nucleosynthesis in Supergiant Stars
It is interesting to note that aluminum and sodium are the only
metallic elements observed thus far in molecular form in the
circumstellar envelope of VY CMa. When considering relative
cosmic elemental abundances, one might expect to observe
magnesium-, calcium-, and potassium-bearing molecules as
Figure 2. Predicted abundances of metal-containing species using an LTE
chemistry model in an oxygen-rich circumstellar environment, based on that of
Tsuji (1973). Conditions were log(Pg ) = 3.0 and C/O = 0.5. The calculations
suggest that hydroxide formation is favored for certain metals (Mg, Al, Ca).
(A color version of this figure is available in the online journal.)
well. The prevalence of Al and Na-containing molecules in VY
CMa may be attributable to nucleosynthesis effects. Although
the main production mechanism of 27 Al and 23 Na is carbon
burning, a small percentage of these nuclei are synthesized
in hydrogen-burning shells of evolved stars (Clayton 2003).
In H-burning shells, proton addition to 26 Mg and 22 Ne leads
to the formation of 27 Al and 23 Na, respectively. Evidence for
this nucleosynthetic process has been found in optical spectra
of supergiant stars, where enhanced sodium and aluminum
abundances appear to be correlated. In M-type supergiants,
Gonzalez & Wallerstein (2000) found sodium enrichments of
up to ten times solar, and aluminum enrichments of up to two
times the solar abundance. Additionally, Takeda & Takada-Hidai
(1994) observed sodium enrichments of ∼6 times the solar value
in A-type supergiants. Similar sodium and aluminum abundance
enhancements may be present in the atmosphere of VY CMa,
which is spectral type M4-M5 (Humphreys et al. 2005). At the
same time, there should be no enrichment of potassium, since
this element is only produced through explosive oxygen burning
(Clayton 2003). These elemental variations appear to manifest
themselves in the molecular species found in VY CMa: AlOH,
AlO, and NaCl, but no KOH or KCl.
Theoretical and observational studies have also shown that
nitrogen has enhanced abundances in supergiant atmospheres.
Spectral line analysis revealed 14 N enrichment of ∼5 times
the solar abundance in supergiants (Takeda & Takada-Hidai
1994; Gonzalez & Wallerstein 2000). The observed nitrogen
enhancement tends to correlate with sodium enhancement, an
No. 1, 2010
EXOTIC METAL MOLECULES IN OXYGEN-RICH ENVELOPES
effect explained by Takeda & Takada-Hidai (1994) as a result
of dredge up of inner products from the CNO and Ne–Na
cycles. The predicted high 14 N abundance could be observable
in supergiants via circumstellar molecular emission. It is notable
that VY CMa has a plethora of N-bearing species (HCN,
HNC, CN, PN, NS, NH3 ; Ziurys et al. 2007, 2009). Further
observations are clearly needed to test this hypothesis.
This research is funded by NSF grants AST-0607803 and
AST-0906534. E.D.T. acknowledges financial support from the
NSF Graduate Research Fellowship Program.
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