EDP Sciences
Free Access
Issue
A&A
Volume 580, August 2015
Article Number A36
Number of page(s) 24
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201525990
Published online 23 July 2015

© ESO, 2015

1. Introduction

The dust-formation sequence in the outflows of oxygen-rich evolved stars is not well understood. It is essential to address which gas-phase species provide the primary seeds. TiO2 is considered an important seed refractory species with possibly higher nucleation rates than SiO (e.g. Jeong et al. 2003; Lee et al. 2015). Moreover, presolar TiO2 grains were tentatively identified by Nittler & Alexander (1999). Since SiO nucleation was recently indicated to be more relevant than previously thought under the relevant pressure and temperature conditions (Nuth & Ferguson 2006; Gail et al. 2013), it is crucial to characterise the role of TiO2. The effect of non-stationarity on the nucleation is unknown. Shocks are, however, known to be present in the upper atmospheres of these evolved stars (e.g. Chiavassa et al. 2011).

Emission from gas phase TiO2 has only been detected towards VY CMa (Kamiński et al. 2013a,b, hereafter K+13a, K+13b). The circumstellar environment of this red supergiant (at 1.2 kpc; Zhang et al. 2012) exhibits a high degree of morphological complexity, from optical to radio wavelengths and on spatial scales from a few to several thousand AU (e.g. Humphreys et al. 2007; Kamiński et al. 2013b; Monnier et al. 2014; Muller et al. 2007; Shenoy et al. 2013; Smith et al. 2001; Ziurys et al. 2007). Recent ALMA observations spatially resolve H2O maser emission, leading to the most accurate determination of the stellar position (Richards et al. 2014, hereafter R+14). O’Gorman et al. (2015, hereafter O+15 describe the submillimeter continuum emission and report on a bright component south-east of the star, indicative of anisotropic mass loss.

TiO2 is expected to be consumed by the nucleation process, but the detection of TiO2 emission by K+13a already suggested that a significant amount could possibly survive the dust formation. K+13a did not spatially resolve TiO2 emission at angular resolutions 1″. The ALMA observations now allow us to characterise the emission and the role of TiO2 in the dust-formation process in more detail.

Table 1

Spectral coverage of the ALMA observations.

thumbnail Fig. 1

TiO2 spectra extracted for a 1′′ diameter aperture around the stellar position. The vertical dashed lines indicate the stellar νLSR of 22 km s-1, the shaded areas the νLSR-ranges from Table 2. We indicate identifications of species other than TiO2 in the panels.

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2. Observations

We retrieved ALMA science verification data on VY CMa from the ALMA archives. The observations and data calibration and reduction are described by R+14. Table 1 shows the spectral coverage and representative rms noise values for the six spectral windows in ALMA’s band 7 (~0.9 mm; ~320 GHz) and one in band 9 (~0.45 mm; ~660 GHz). With projected baselines of 14 m up to 2.7 km, the spatial resolution at ~320 GHz and 658 GHz is ~ and ~, respectively, and the maximum recoverable scales are 8.̋3 and 4.̋0. The synthesised beam sizes are obtained using natural weighting.

The data reduction and quality of the continuum emission images at 321 GHz and 658 GHz are discussed in detail by R+14 and O+15. No imaging artefacts are expected to arise from the array configuration owing to the excellent coverage of the visibility plane.

We note that the spectral window 312−314 GHz suffered from poor continuum subtraction owing to line crowding and a consequent lack of line-free spectral range. This causes a fraction of the continuum emission to leak into the channel maps of the TiO2 lines at ~312 GHz (Figs. A.4A.6). The emission seen at the position of clump C can be entirely attributed to the continuum itself, i.e. we can rule out molecular contribution at this position. The lines presented in Fig. 1 and Table 2, and consequently also the reported peak and integrated intensities, were corrected for this effect on a line-to-line basis.

The imaged TiO2 lines in the 312−314 GHz spectral window also show a contribution north-north-east and south-south-west of the star. These features are artefacts of the cleaning procedure likely caused by imperfect phase corrections on a number of intermediate baselines. The phase corrections were transferred from the self-calibrated 325 GHz maser line located in the atmospheric absorption region.

3. Results

We analysed spectra extracted for a 1′′ diameter region around the stellar position. We show below that no TiO2 emission is detected beyond this aperture. TiO2 identifications are based on the Cologne Database for Molecular Spectroscopy (CDMS; Müller et al. 2001, 2005; Brünken et al. 2008). We detect 15 lines with upper-level energies Eup/k in the range 48−676 K and signal-to-noise ratios S/N ≈ 5−17 at velocity resolutions 0.9−7.6 km s-1 (Table 2, Fig. 1). Of the TiO2 lines detected by K+13a; K+13b only those at 310.55 GHz and 310.78 GHz are observed with ALMA. The peak fluxes of the ALMA and SMA detections are consistent within the uncertainties. Moreover, of the other 13 lines of TiO2 detected with ALMA, none were detected in the SMA survey, owing to a noise level in the ALMA data that was approximately 10 times lower. We detect no TiO2 emission at ~660 GHz owing to the higher rms noise (Table 1). We detect no isotopic variants of Ti, consistent with the solar isotopic abundance ratios.

We expect no large flux losses in the ALMA observations of TiO2, given the ~8′′ recoverable scale at ~320 GHz. The presence of large-scale emission with low surface brightness cannot be entirely excluded, but is unlikely and will not change our main conclusions.

Table 2

Overview of detected TiO2 lines.

Blending, proximity to strong lines, line crowding, and intrinsic line-shape irregularities complicate the identification of TiO2 emission. The lines at 310.55 GHz and 311.46 GHz are only partially covered in the observations; we believe the former to be a firm detection and the latter to be tentative. In both cases, no other candidate could be identified. Uncertainties on the relevant TiO2 frequencies are <2 MHz, except for the transition at 311.462 GHz, where it is 5.555 MHz (Brünken et al. 2008).

The TiO2 lines detected with ALMA exhibit broad line profiles, as do those detected by K+13a. Assuming1 a stellar νLSR of 22 km s-1, we find that the emission is very asymmetric in velocity space, ranging typically from ~−15 km s-1 to ~60 km s-1 (Fig. 1). The apparent extension of the blue wing of some lines to ~−45 km s-1 is likely coincidental and due to line blending, but we cannot exclude an actual TiO2 contribution. Blend candidates are indicated in Fig. 1 and Table 2. The lines with Eup/k = 48 K2, 58 K, 182 K, 532 K exhibit features in their red wings that extend to νLSR ≈ 105 km s-1. A similar high-velocity outflow is also visible for some lines presented by K+13a.

thumbnail Fig. 2

TiO2 morphology. Colour maps of emission at 310.78 GHz integrated over the νLSR-ranges indicated at the top left of each panel, cut off at 3σ. Red contours show the 321 GHz continuum at [3, 20, 40, 60, 80] σ; green contours show HST emission at [3, 5, 7, 10, 20, 30, 40, 50, 100, 200] σ (Smith et al. 2001). In the first panel we mark the position of the star (+, VY; black) and of the continuum component (x, C; red) to the south-east (O+15; R+14), and the position and approximate extent of the south-west clump (SW, dashed 1′′ diameter circle; red Shenoy et al. 2013). The apparent north-south emission is thought to arise from dynamic-range limitations in the peak channels.

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3.1. Spatial distribution

The detected TiO2 emission is, for the most part, spatially resolved by the ALMA observations with a beam (see Figs. A.1–A.16), but several transitions show spatially unresolved emission at a 2 km s-1 velocity resolution. We note a very complex behaviour of the emission peaks, with some only appearing in one channel and not in the neighbouring channels. The often spatially unresolved (145 AU) peaks in the different velocity channels of all lines imply a clumpy and/or anisotropic wind. The maps for 310.55 GHz (Eup/k = 181 K) and 310.78 GHz (Eup/k = 182 K) reveal that these two lines behave almost identically. Corresponding velocity channels (Figs. A.1, A.2) trace roughly the same regions of the circumstellar environment and exhibit similar intensities, implying that these two lines are very tightly coupled in their excitation. Figure 2 shows the morphology of the TiO2 emission at 310.78 GHz and the positions of the star (VY), the continuum component C (O+15; R+14), and the south-west clump detected at λ ~ 1−5μm at ~1′′ from the star (Smith et al. 2001; Humphreys et al. 2007; Shenoy et al. 2013).

Given the similar energy levels, quantum numbers, and Einstein-A coefficients of the transitions at 310.55 GHz, 310.78 GHz, and 324.49 GHz, one expects similar line intensities and spatial distributions. However, although the channel-to-channel peaks of the 324.49 GHz emission (Fig. A.11) correspond quite well to those of the other two, which strengthens its identification as TiO2, its intensity is clearly lower. It is hard to explain this discrepancy, but we note that as a consequence of the sensitivity of the atmospheric transmission at ~325 GHz to the atmospheric water-vapour content the rms noise is ~4 times higher and the flux calibration could be compromised.

The emission at 322.61 GHz, 322.33 GHz, and 324.96 GHz behaves similarly but with weaker and spatially somewhat more confined red-wing emission. Unfortunately, many of the TiO2 lines are blended (both in frequency and spatially) with emission from other species.

The discussion below is focussed on the morphology at 310.78 GHz with Fig. 2 as a visual guide. Overall, the detected TiO2 emission moves from west to east, across the stellar position, with velocities evolving from reddest to bluest. All lines show emission close to the star at the stellar νLSR. In particular, all lines with Eup/k ≳ 360 K are centred on VY and emit mainly across νLSR ≈ 0−40 km s-1 (Fig. 1), implying that these trace a central part of the outflow that may be accelerating.

At νLSR ≥ 44 km s-1

the emission is mainly situated west of VY with a marginal contribution close to it. The emission is highly variable with νLSR, doubly peaked in most velocity channels, and shows a hook-like feature at its western edge. The latter could relate to the north-west knot defined by Humphreys et al. (2007) based on HST observations, though more analysis is needed to investigate the nature of this feature.

At 22 < νLSR< 44 km s-1

we find multiple emission peaks in most velocity channels, with those closest to VY slightly brighter than the west offset peaks. The integrated emission is elongated along a direction roughly parallel to the axis connecting the peak position of C and VY, at a PA 125°.

At 18 ≤ νLSR ≤ 22 km s-1

the transitions at ~310 GHz show a bright tail extending south-west of VY at a PA 220°, i.e. almost perpendicular to the axis connecting VY and C. This tail reaches ~ 1″ away from VY, out to the south-west clump of Shenoy et al. (2013) and agrees very well with the features detected by e.g. Smith et al. (2001) at wavelengths λ ~ 1−2.14μm. The emission at 310.78 GHz extends slightly further south-west than that at 310.55 GHz. Since the most extended emission exceeds 5σ, we suggest that the difference is real and that clumpiness in the outflow strongly influences the excitation of individual lines.

At 312.73 GHz and 312.82 GHz

we find signs of emission in the south-west tail, but these are likely artefacts from the cleaning procedure (see Sect. 2). At 311.46 GHz we see emission within the spatial region where the wouth-west tail is located, although at higher νLSR than the transitions at ~310 GHz. This could be due to the south-west tail covering a wider velocity range than reported above, by a line blend, or a misidentification of the line as TiO2. We unfortunately do not have information for νLSR< 22 km s-1 for this line. No other transition shows detectable emission within the south-west tail, but most show a slight bulge of emission at ~ south-west of the star, at the base of the south-west tail. Figure A.17 shows a comparison for all lines to the south-west tail observed at 310.78 GHz.

At νLSR< 18 km s-1

the high-intensity emission is situated entirely east of VY, elongated, and oriented at ~15°−25° east from north. With bluer velocities the emission moves towards C and then appears to break up with a northern peak brighter than the southern one (e.g. Fig. A.2). Remarkable is that the low-intensity component of the 310.78 GHz transition closely resembles the scattered light at 1 μm. We find this strong correspondence at these blue velocities for no other TiO2 transition. If the high- and low-intensity components have different intrinsic wind velocities, they could be spatially separated and trace different parts of the outflow.

3.2. Excitation conditions

We investigate the excitation of TiO2 lines via a rotational diagram analysis (Fig. 3). Intensities, source sizes, and rms noise values are taken from Table 2. Severely blended or only partially covered lines are excluded from the analysis. We derive a source-averaged column density Ncol = 5.65 ± 1.33 × 1015 cm-2 and a rotational temperature Trot = 198.0 ± 28.5 K, in agreement with K+13a. We note that the kinetic temperature in the excitation region of TiO2 varies from more than 1000 K down to ~100 K (Decin et al. 2006). Assuming an average mass-loss rate of 2 × 10-4M yr-1 (e.g. De Beck et al. 2010), an average velocity of 20 km s-1, and an 0.̋9 diametric extent, at 1.2 kpc, we find an abundance TiO2/ H2 ≈ 3.8 ± 0.9 × 10-8.

thumbnail Fig. 3

Rotational diagram. Lines indicated in red are blended or only partially covered in the observations and are excluded from the fitting procedure. All intensities, source sizes, and rms noise values are taken from Table 2. The fit results and uncertainties are shown with the blue and grey dashed lines and are indicated at the top right.

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The high dipole moment of TiO2 (6.33 Debye; Wang et al. 2009) supports efficient radiative excitation and Trot could hence reflect an average continuum brightness temperature as opposed to a gas kinetic temperature in the case of collisional excitation. Additionally, the large dipole moment induces electron-TiO2 collision rates large enough to exceed H2–TiO2 collision rates if the fractional ionisation exceeds a few 10-6. We could not find a value for VY CMa, but based on the result ne/nH = 3.8 × 10-4 for the red supergiant α Ori (Harper et al. 2001) this type of excitation could be relevant3. We discuss the competition between collisional excitation and radiative excitation in more detail below, in Sect. 4.1.

4. Discussion

4.1. Excitation of titanium dioxide

Table 3

Summary of properties of selected transitions of TiO2.

Many effects need to be taken into account in a full treatment of the excitation and radiative transfer of rotational transitions of TiO2. Important molecular data are lacking: although the rotational energy levels and radiative data are well determined for TiO2 in CDMS (Müller et al. 2001, 2005; Brünken et al. 2008), no rates exist for excitation of TiO2 by hydrogen-impact nor have the vibration-rotation spectra been fully analysed.

For illustration of the transition rates for TiO2, consider two of the observed rotational transitions at 312.248 and 324.965 GHz with properties summarised in Table 3. Collisional excitation at the kinetic temperature of the gas can dominate only when the downward rate of collision-induced transitions greatly exceeds the downward rates of radiative transitions for each upper state. This condition can be translated to a critical density n0 = ∑ Au,ℓ/q0, where Au,ℓ is the spontaneous transition probability of each transition from upper state u to lower state . To estimate this density, we assume a characteristic4 quenching rate coefficient q0 = 10-10 cm3 s-1 for rotational transitions induced by collisions with neutral species H or H2. Values for n0 are listed in Table 3. Assuming a mass-loss rate of 10-4M yr-1, such densities are however reached only at distances from the star lower than ~0.̋065, whereas TiO2 is excited over a much larger region, where the density quickly decreases to a few 105 cm-3.

In addition, collision rates for electron impact have been computed in the Born approximation. Because the electric dipole moment of TiO2 is so large, electron-impact rates are expected to show a strong propensity for radiatively allowed transitions. Computed rates are likely to be accurate within 50%. Under an assumed very low fractional electron density of 10-7, the collision-induced downward transition rate for the 312 GHz transition is for 5% due to electron collisions. Assuming the fractional ionisation reported for α Ori (3.8 × 10-4) electron collisions would completely dominate over neutral (hydrogen) collisions by a factor of the order of 100 or more and might compete well with infrared pumping.

We note that the upper state of the 312 GHz transition decays mainly by the observed transition itself, while the upper state of the high-excitation 324 GHz transition is depopulated more rapidly by submm-wave transitions at 821 and 1235 GHz. As a consequence, a much higher density would be required to excite the 324 GHz line by collisions than the 312 GHz line.

The continuum intensity of VY CMa is so large at infrared and submm wavelengths that radiative excitation (pumping) must be taken into account. If the observed continuum flux of VY CMa is assumed to arise within a diameter region (O+15) and to be diluted by a geometrical factor 1/9 over the extent of the observed TiO2 emission, then the average Planckian radiation brightness temperatures are estimated to be Trad ≥ 25 K at 312 to 324 GHz. The corresponding pumping rates (absorption and stimulated emission) in this radiation field can be expressed as

for each radiative transition that connects states of interest. The basic radiative data for TiO2 are collected in Table 3. Although the fundamental bands of the three vibrational modes have not been fully analysed rotationally, the band frequencies and band strengths are approximately known (Grein 2007). In the adopted continuum model, the infrared intensity is high enough to drive absorption in vibration-rotation lines at rates of the order of 0.01 to 1.0 s-1. In order for collisional excitation in a pure rotational transition u to compete with infrared pumping in VY CMa, a density much greater than

would be required. The radiative rates in Table 3 suggest that radiative excitation is likely to be very important for TiO2 in VY CMa. Therefore, the rotational temperature derived in Sect. 3.2 might have no direct relationship to the kinetic temperature. Because the observed transitions in the present study span a wide range of excitation energies, the simple rotation-diagram analysis still provides a useful first estimate of the molecular column density and abundance.

4.2. Outflow components

From the comparison to the continuum at ~321 GHz, we derive that TiO2 is excited in the directions with lower dust densities. The absence of detected TiO2 emission north of the star could then point to efficient obscuration of this part of the outflow, in line with the observations of e.g. Smith et al. (2001). Attenuation of the stellar radiation field to the east and west of the star is limited. From this, we do not expect an equatorial enhancement of the mass-loss rate, since this would likely have induced more efficient dust formation, which is not seen in the continuum.

The TiO2 emission traces multiple wind components. We find a red outflow to the west and a blue outflow to the east. With the clear exception of the interaction of the TiO2 gas with clump C in the east, the two seem roughly symmetric around the star and aligned with the axis connecting VY and C. We rule out an equatorially enhanced environment such as an expanding disk or ring, based on the spatial distribution of the TiO2 emission at different νLSR. We rather suggest an accelerating bipolar-like outflow at lower densities. We also find a predominantly blue south-west outflow, connecting the star and the south-west clump, approximately perpendicular to the VY – C connecting axis. We find no north-east counterpart in TiO2 emission, likely implying that the south-west tail is indeed caused by an event in one preferred direction, as opposed to a bipolar event.

4.3. Interaction of blue outflow with clump C

Whereas the H2O maser emission in the “valley” between VY and C implies that C is close to or in the plane of the sky (R+14), the observations suggest that the TiO2 gas breaks up around C while moving towards the observer, placing C – at least partially – in front of the plane of the sky. We therefore deem it likely that the H2O masers and the TiO2 emission probe parts of the outflow east of VY with different physical properties. Whereas the masers are probably excited through shocks at high densities, TiO2 is more likely excited through radiation, at lower outflow densities. In the denser regions, TiO2 might not be excited and/or it might be efficiently depleted from the gas phase. The latter is, however, less likely (see below). We therefore suggest that TiO2 traces the blue-shifted wind to the east of VY with lower densities which runs into and curves around C.

4.4. Titanium dioxide and scattered light

K+13b reported emission offset by ~1″ south-west from the central molecular emission for multiple species. However, owing to lower sensitivity they did not find indications for this in the TiO2 emission, whereas we clearly detect the south-west tail at ~310 GHz (see Sect. 3.1).

The likelihood of radiative excitation of TiO2 and the agreement between the south-west tail in the TiO2 emission and the scattered stellar light at 1 μm from Smith et al. (2001) suggest the presence of a tail of gas and dust lit up by a stellar radiation field less attenuated than at other position angles. The bulge south-west of the star seen for most TiO2 transitions is consistent with this. We infer a lower degree of attenuation and from that a lower (dust) density south-west of the star. We speculate that the suggested localised ejection leading to the formation of the south-west clump (Shenoy et al. 2013) could have created a cavity in the dense material close to the star.

Given the agreement with the blueshifted TiO2 emission, the outflow traced by the scattered light is likely oriented out of the plane of the sky. The lack of consistent νLSR coverage over the different TiO2 transitions, however, further complicates the three-dimensional and kinematic constraints. In-depth analysis of all other emission lines detected with ALMA is needed to constrain the basic properties of the south-west tail.

4.5. Titanium dioxide and dust formation

If the nucleation of TiO2 can occur at 2000 K (e.g. Lee et al. 2015), one expects TiO2 depletion from the gas phase initiated close to the star, i.e. within a few stellar radii. From the ALMA observations O+15 put an upper limit to the dust-condensation radius of 10 stellar radii. The TiO2 emission is present close to the star and out to 0.̋45 in directions without extended continuum emission. Where TiO2 and the bulk dust coexist, the emission extends to ~0.̋20, well beyond the dust-condensation radius. Since the TiO2 emission is seen at radial distances from the star far beyond the dust-condensation region, and along directions where dust continuum is observed, we claim that TiO2 is not a tracer for low grain-formation efficiency. The strong correspondence between the TiO2 emission and the south-west tail of dust-scattered stellar light (see Sect. 3.1 and Fig. 2) supports this. We also considered the high derived column density and suggest that TiO2 plays only a minor role as a primary dust seed around VY CMa.

Since the ALMA observations covered no transitions of TiO, we cannot expand the discussion of K+13a on the relation between TiO, TiO2, and the dust.

5. Conclusion

We detect 15 transitions of TiO2 in high-resolution ALMA observations of the red supergiant VY CMa. The main emission region spans ~0.̋9 (~1080 AU) in a roughly east-west oriented direction, centred on the star. The behaviour of the gas is complex throughout νLSR-space with bright peaks appearing only in very confined νLSR-ranges, implying that the outflow is very clumpy, on spatial scales not resolved by the current observations with a beam. We observe a tail of TiO2 emission extending out to ~1′′ south-west of the star, consistent with structures seen in the optical and near-infrared. It is oriented out of the plane of the sky and mainly covers projected velocities of a few km s-1, but reaches up to ~40 km s-1 in some cases. We suggest that the TiO2 in this tail is illuminated by stellar radiation penetrating through a low-density cavity in the south-west part of the circumstellar environment of the star, potentially created by the ejection that led to the south-west clump. Within a bipolar-like TiO2 outflow, the blue-shifted emission exhibits a strongly different orientation and behaviour than the red-shifted emission, suggesting that the stellar wind runs into the large body of dust situated ~0.̋335 (~400 AU) south-east of the star.

We suggest that TiO2 might play only a minor role in the dust-condensation process in the complex outflow around VY CMa, and potentially also around other oxygen-rich evolved stars with extreme mass outflows. High-resolution imaging is however still needed to correlate the emission of TiO2 with that of TiO, and to further investigate the relative importance of silicon, titanium, and other metals in the dust condensation.


1

See K+13b for a discussion of νLSR.

2

Continuum subtraction is hampered by the high line density. The red wing of the line is affected, but still shows the feature they have in common.

3

VY CMa is of spectral type M2.5-M5e Ia (Houk & Smith-Moore 1988), α Ori of spectral type M1-2Ia-ab (Keenan & McNeil 1989).

4

Such a value is typical of the largest downward collisional rates for H2 collisions with a heavy, polar molecule. For example, accurate collision rates have been computed by Cernicharo et al. (2011) for H2 on SO2, a heavy molecule with a relatively large dipole moment (1.63 Debye). The SO2 quenching rates at low temperature are ~ 2 × 10-10.

Acknowledgments

This paper makes use of the following ALMA data: ADS/JAO.ALMA2011.0.00011.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. W.V. and E.O.G. acknowledge support from the ERC through consolidator grant 614264. M.M. has received funding from the People Programme (Marie Curie Actions) of the EU’s FP7 (FP7/2007-2013) under REA grant agreement No. 623898.11.

References

Online material

Appendix A: Maps of titanium dioxide emission

Figures A.1 to A.15 show channel maps of the detected TiO2 emission lines (Table 2, Fig. 1) at a velocity resolution of 2 km s-1, covering the range −14 ≤ νLSR ≤ 78 km s-1. This νLSR-range covers the bulk of all TiO2 emission; emission at

more extreme velocities is no longer visible in the channel maps. Figure A.16 shows integrated-intensity maps for all listed lines, covering the νLSR-ranges indicated in Table 2. Figure A.17 shows a comparison of the TiO2 line emission to the south-west tail detected at 310.78 GHz, which is discussed in Sects. 3.1 and 4.4.

thumbnail Fig. A.1

Channel maps of the TiO2 emission at 310.55 GHz, at a 2 km s-1 velocity resolution. Black contours show the continuum measured with ALMA at 321 GHz (O+15; R+14). The stellar position is indicated with a white cross. Spatial scales are indicated in the top left panel and are the same for all panels. The colour scale starts at the 3σ level and is plotted as the square root of the flux for increased contrast.

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thumbnail Fig. A.2

Same as Fig. A.1, but for TiO2 emission at 310.78 GHz.

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thumbnail Fig. A.3

Same as Fig. A.1, but for TiO2 emission at 311.46 GHz.

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thumbnail Fig. A.4

Same as Fig. A.1, but for TiO2 emission at 312.25 GHz.

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thumbnail Fig. A.5

Same as Fig. A.1, but for TiO2 emission at 312.73 GHz.

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thumbnail Fig. A.6

Same as Fig. A.1, but for TiO2 emission at 312.82 GHz.

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thumbnail Fig. A.7

Same as Fig. A.1, but for TiO2 emission at 321.40 GHz. Artefacts in channels with νLSR ≥ 55 km s-1 are due to the presence of strong SO2 emission. See also Fig. 1.

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thumbnail Fig. A.8

Same as Fig. A.1, but for TiO2 emission at 321.50 GHz.

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thumbnail Fig. A.9

Same as Fig. A.1, but for TiO2 emission at 322.33 GHz.

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thumbnail Fig. A.10

Same as Fig. A.1, but for TiO2 emission at 322.61 GHz.

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thumbnail Fig. A.11

Same as Fig. A.1, but for TiO2 emission at 324.49 GHz.

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thumbnail Fig. A.12

Same as Fig. A.1, but for TiO2 emission at 324.96 GHz.

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thumbnail Fig. A.13

Same as Fig. A.1, but for TiO2 emission at 325.32 GHz.

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thumbnail Fig. A.14

Same as Fig. A.1, but for TiO2 emission at 325.50 GHz.

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thumbnail Fig. A.15

Same as Fig. A.1, but for TiO2 emission at 325.60 GHz.

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thumbnail Fig. A.16

Integrated-intensity maps of TiO2 emission. Colour maps show the intensity integrated over the νLSR-ranges indicated in Table 2 and Fig. 1 and cut off at 3σ. Contours show the ALMA 321 GHz continuum. Labels indicate the positions of the star (+, VY) and the continuum component (x, C) to the south-east (O+15; R+14), and the position and approximate extent of the south-west clump of Shenoy et al. (2013, SW and a dashed 1′′. The presence of cleaning artefacts, mainly at ~312 GHz, is addressed in Sect. 2.

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thumbnail Fig. A.17

Comparison of TiO2 emission lines listed in Table 2 to south-west tail at 310.78 GHz (red contours) and HST image (black contours). Emission integrated over 19 ≤ νLSR ≤ 22 km s-1 plotted at > 3σ (colour scale). Emission of 311.46 GHz was omitted since the relevant νLSR-range is not covered by the observations.

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All Tables

Table 1

Spectral coverage of the ALMA observations.

Table 2

Overview of detected TiO2 lines.

Table 3

Summary of properties of selected transitions of TiO2.

All Figures

thumbnail Fig. 1

TiO2 spectra extracted for a 1′′ diameter aperture around the stellar position. The vertical dashed lines indicate the stellar νLSR of 22 km s-1, the shaded areas the νLSR-ranges from Table 2. We indicate identifications of species other than TiO2 in the panels.

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In the text
thumbnail Fig. 2

TiO2 morphology. Colour maps of emission at 310.78 GHz integrated over the νLSR-ranges indicated at the top left of each panel, cut off at 3σ. Red contours show the 321 GHz continuum at [3, 20, 40, 60, 80] σ; green contours show HST emission at [3, 5, 7, 10, 20, 30, 40, 50, 100, 200] σ (Smith et al. 2001). In the first panel we mark the position of the star (+, VY; black) and of the continuum component (x, C; red) to the south-east (O+15; R+14), and the position and approximate extent of the south-west clump (SW, dashed 1′′ diameter circle; red Shenoy et al. 2013). The apparent north-south emission is thought to arise from dynamic-range limitations in the peak channels.

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In the text
thumbnail Fig. 3

Rotational diagram. Lines indicated in red are blended or only partially covered in the observations and are excluded from the fitting procedure. All intensities, source sizes, and rms noise values are taken from Table 2. The fit results and uncertainties are shown with the blue and grey dashed lines and are indicated at the top right.

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In the text
thumbnail Fig. A.1

Channel maps of the TiO2 emission at 310.55 GHz, at a 2 km s-1 velocity resolution. Black contours show the continuum measured with ALMA at 321 GHz (O+15; R+14). The stellar position is indicated with a white cross. Spatial scales are indicated in the top left panel and are the same for all panels. The colour scale starts at the 3σ level and is plotted as the square root of the flux for increased contrast.

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In the text
thumbnail Fig. A.2

Same as Fig. A.1, but for TiO2 emission at 310.78 GHz.

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In the text
thumbnail Fig. A.3

Same as Fig. A.1, but for TiO2 emission at 311.46 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.4

Same as Fig. A.1, but for TiO2 emission at 312.25 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.5

Same as Fig. A.1, but for TiO2 emission at 312.73 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.6

Same as Fig. A.1, but for TiO2 emission at 312.82 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.7

Same as Fig. A.1, but for TiO2 emission at 321.40 GHz. Artefacts in channels with νLSR ≥ 55 km s-1 are due to the presence of strong SO2 emission. See also Fig. 1.

Open with DEXTER
In the text
thumbnail Fig. A.8

Same as Fig. A.1, but for TiO2 emission at 321.50 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.9

Same as Fig. A.1, but for TiO2 emission at 322.33 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.10

Same as Fig. A.1, but for TiO2 emission at 322.61 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.11

Same as Fig. A.1, but for TiO2 emission at 324.49 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.12

Same as Fig. A.1, but for TiO2 emission at 324.96 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.13

Same as Fig. A.1, but for TiO2 emission at 325.32 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.14

Same as Fig. A.1, but for TiO2 emission at 325.50 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.15

Same as Fig. A.1, but for TiO2 emission at 325.60 GHz.

Open with DEXTER
In the text
thumbnail Fig. A.16

Integrated-intensity maps of TiO2 emission. Colour maps show the intensity integrated over the νLSR-ranges indicated in Table 2 and Fig. 1 and cut off at 3σ. Contours show the ALMA 321 GHz continuum. Labels indicate the positions of the star (+, VY) and the continuum component (x, C) to the south-east (O+15; R+14), and the position and approximate extent of the south-west clump of Shenoy et al. (2013, SW and a dashed 1′′. The presence of cleaning artefacts, mainly at ~312 GHz, is addressed in Sect. 2.

Open with DEXTER
In the text
thumbnail Fig. A.17

Comparison of TiO2 emission lines listed in Table 2 to south-west tail at 310.78 GHz (red contours) and HST image (black contours). Emission integrated over 19 ≤ νLSR ≤ 22 km s-1 plotted at > 3σ (colour scale). Emission of 311.46 GHz was omitted since the relevant νLSR-range is not covered by the observations.

Open with DEXTER
In the text

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