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.
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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 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 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 chopper-wheel 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 = 07h20m54.7s 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.
Table 1. Line Parameters for AlOH in VY CMa
| Transition | Frequency | Observedb | Modelc | |||||
|---|---|---|---|---|---|---|---|---|
| (MHz) | θb | VLSR | TRb | ΔV1/2 | ∫TRdV | ∫TRdV | ||
| ηa | ('') | (km s−1) | (mK) | (km s−1) | (K km s−1) | (K km s−1) | ||
| J = 9 → 8 | 283,253.9 | 0.78 | 27 | 21 ± 2.1 | 3.8 ± 0.7 | 21.2 ± 2.1 | 0.063 ± 0.006 | 0.085 |
| J = 7 → 6 | 220,330.9 | 0.78 | 34 | 19 ± 2.7 | 2.0 ± 0.4 | 16.3 ± 2.7 | 0.036 ±0.004 | 0.035 |
| J = 5 → 4 | 157,391.0 | 0.72 | 40 | 19 ± 3.8 | 1.5 ± 0.5 | 23 ± 8 | 0.02 ± 0.005 | 0.013 |
Notes. aηb for SMT data and ηc for 12 m data (see the text). bReported error ranges are 2σ. cIntegrated line intensities predicted by the best-fit radiative transfer model (see the text).
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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 1Σ+ ground state. At lower J transitions, AlOH exhibits a quadrupole hyperfine pattern due to the I = 5/2 spin of the 27Al 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.
Figure 1. Spectra of the J = 9 → 8 and 7 → 6 transitions of AlOH (X1Σ+) 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 13CO 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.
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Standard image High-resolution imageSpectral 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 13CO (see Figure 1). In addition, emission lines from the JKa,Kc = 160,16 → 151,15 transitions of both 32SO2 and 34SO2 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 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 
. 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 0.13'' (11 R*). A value of rinner ∼ 5 × 1014 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.
4.2. Metal Hydroxides: A New Class of Circumstellar Molecules?
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.
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).
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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 (θs = 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.
Table 2. Abundances of Al, Na, and K-bearing Molecules in VY CMa
| Species | Observed Abundancea | LTE-predicted Abundance |
|---|---|---|
| AlOH | 1 × 10−7 | 2 × 10−6 |
| NaOH | ≤ 3 × 10−9 | 1 × 10−7 |
| KOH | ≤ 1 × 10−9 | 8 × 10−9 |
| AlCl | ≤ 5 × 10−8 | 4 × 10−8 |
| NaCl | 4 × 10−9 | 1 × 10−7 |
| KCl | ≤ 4 × 10−10 | 6 × 10−8 |
| AlO | 6 × 10−9 | 3 × 10−10 |
Note. aBased on ARO observations of multiple transitions of each molecule.
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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 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 sodium-containing 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 Al2O3 at 1700 K (Lodders & Fegley 1999; Gail & Sedlmayr 1998), although it is unclear how AlOH condenses into grains. Furthermore, H2O 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 well. The prevalence of Al and Na-containing molecules in VY CMa may be attributable to nucleosynthesis effects. Although the main production mechanism of 27Al and 23Na 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 26Mg and 22Ne leads to the formation of 27Al and 23Na, 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 14N 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 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 14N 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.