First Images of the Molecular Gas around a Born-again Star Revealed by ALMA

From the ALMA observations, we obtained the first images of the submillimeter continuum and molecular emission of the material ejected by the born-again star V 605 Aql. The continuum emission appears as a broken and clumpy ring-like structure with an average diameter of ≈03, although there is fainter emission extending over a region of ≈07 in diameter. The ring-like structure is slightly elongated in the east–west direction (see Figure 1). In the right panel of Figure 1 the ALMA continuum image is compared to the [O iii] emission from HST observations. The HST image was shifted 03 in decl. to align the geometric center of the continuum ring, (J2000) R.A. = 19^h18^m20538, decl. = +1°46'58741, with the [O iii] emission peak, which is approximately the location of the central star found by Clayton et al. (2013). The continuum emission is less extended than the [O iii] emission and below we propose that it is associated with the dark band seen at optical wavelengths.

Zoom In Zoom Out Reset image size

Apart from the continuum image that was created by combining the emission in the four spws, continuum images of each individual spw were also created. As mentioned in Section 2, the central frequencies of the spws are 343.031, 344.989, 355.031, and 356.911 GHz, and their corresponding continuum emission fluxes are 32.2, 33.2, 36.0, and 37.1 mJy. A power-law fit to the spectral energy distribution (SED) in the frequency range of the ALMA observations gives S_ν ∝ ν^3.3±0.3 (see Figure 2). It is well known that at submm wavelengths dust is the most important source of continuum emission, whose flux exhibits a power-law dependence with frequency as S_ν ∝ ν^2+β , where β is the emissivity spectral index of the dust grains (Hildebrand 1983). Thus, the power-law dependence of the ALMA continuum corresponds to dust emission with an emissivity spectral index β = 1.3 ± 0.3. Figure 2 shows the SED of V 605 Aql including observations within the wavelength range from ∼1 μm to ∼1 mm. Similarly to Clayton et al. (2013), we did a three-component fit to the data points, including our ALMA observations, but we used modified blackbody curves with an emissivity spectral index β = 1.3. The 75 K component fits very well the continuum emission at ∼350 GHz observed with ALMA. The dust mass can be estimated assuming optically thin emission and using the following expression:

$$\begin{eqnarray*}&&{M}_{\mathrm{dust}}=\displaystyle \frac{{S}_{\nu }\,{D}^{2}}{{\kappa }_{\nu }\,{B}_{\nu }({T}_{\mathrm{dust}})},\end{eqnarray*}$$

where S_ν is the flux density of the continuum emission, D is the distance to the source, κ_ν is the dust absorption coefficient and B_ν (T) is the Planck function. The composition of the dust grains in V 605 Aql is considered to be amorphous carbon (see, e.g., Clayton et al. 2013, and references therein). The value for the dust absorption coefficient of amorphous carbon reported in the literature ranges from κ(ν = 350 GHz) ∼ 1.3 cm² gm⁻¹ up to κ(ν = 350 GHz) ∼ 60 cm² gm⁻¹ (e.g., Draine & Lee 1984; Martin & Rogers 1987; Ossenkopf & Henning 1994; Zubko et al. 1996; Mennella et al. 1998; Suh 2000), with values at the low end being more common. Thus, for a dust temperature T_dust = 75 K and a distance to the source D = 4 kpc, the resulting dust mass varies in the range M_dust ∼ 0.2–8 × 10⁻³ M_⊙, which is in agreement with the values, re-scaled by the distance, obtained by Clayton et al. (2013) and Koller & Kimeswenger (2001). It is clear that the adoption of different values for the dust absorption coefficient and distance to the source lead to significant differences in the derived values of the dust mass. Further work on determining the detailed characteristics of the dust grains in the ejecta of V 605 Aql is necessary to better constrain the dust mass.

Zoom In Zoom Out Reset image size

Emission from the CO(J = 3 → 2), HCN(J = 4 → 3), H¹³CN(J = 4 → 3) and HCO⁺(J = 4 → 3) lines was also detected from the ALMA observations. Figure 3 shows the spectra of the emission integrated over a circular region centered at the position of the central star and with a diameter of 12. The peak flux of the CO(J = 3 → 2) line agrees well with the APEX observations presented by Tafoya et al. (2017), indicating that there is no missing flux from the interferometric observations. The lines exhibit a double peak profile and the emission extends over a relatively broad velocity range. The HCN(J = 4 → 3) line seems to extend over a broader velocity range than the rest of the lines. The dip at the center of the lines coincides with the systemic velocity, v_sys,LSR = 96 km s⁻¹, derived from the APEX observations. The velocity-integrated intensity of the redshifted side of the lines is lower than that of the blueshifted side, although the peak emission is comparable.

Zoom In Zoom Out Reset image size

Figure 4 shows the velocity-integrated emission, zeroth moment, images of the emission for the different molecular species. The zeroth moment images were obtained by integrating the emission of the corresponding spectral line for each pixel along the velocity-axis within the velocity range −v_max ≲ v_offset ≲ +v_max, where v_offset is defined as v_offset = v_LSR ⁷ − v_sys,LSR, and ${v}_{\max }$ is the maximum velocity offset of the line. As it can be seen from Figure 3, ${v}_{\max }$ differs from one molecular species to another, varying from ∼150 km s⁻¹ to ∼200 km s⁻¹, the latter corresponding to the HCN(J = 4 → 3) line.

Zoom In Zoom Out Reset image size

The molecular emission also exhibits a clumpy ring-like structure and its spatial extent is somehow similar to that of the continuum emission, shown as white contours in the panels of Figure 4. Particularly, the CO(J = 3 → 2) line is slightly more extended than the continuum emission and it is elongated in a different direction (see Section 3.2). The molecular ring is brighter in the southern edge where the continuum emission is brighter too. We calculated the mass of the gas for the different molecular species assuming optically thin emission, LTE conditions, and adopting a distance of 4 kpc. For these calculations, we followed a formalism similar to the one presented by Mangum & Shirley (2015) and used values of the Einstein A-coefficients, statistical weights, and partition functions from the Cologne Database for Molecular Spectroscopy (Müller et al. 2001, 2005). For an excitation temperature of T_ex = 75 K, which is the temperature of the dust component that fits the ALMA continuum, the derived values of the molecular masses are M_CO = 1.05 ± 0.01 × 10⁻⁵ M_⊙, ${M}_{\mathrm{HCN}}=9.1\pm 0.1\times {10}^{-9}$ M_⊙, ${M}_{{{\rm{H}}}^{13}\mathrm{CN}}=1.6\pm 0.1\times {10}^{-9}$ M_⊙, and ${M}_{{\mathrm{HCO}}^{+}}=4.5\pm 0.1\times {10}^{-9}$ M_⊙. Thus, the main component of the molecular material is CO, perhaps even exceeding H₂ (although empirical determination of the H₂ content in born-again PNe is lacking).

The velocity field of the molecular gas is obtained by calculating the intensity-weighted spectral moment distribution, first moment, of the emission. We created first moment images from the emission of all the detected molecules. Since the first moment of all the detected molecules exhibits similar overall spatio-kinematical distributions, we show in Figure 5 the results for the CO(J = 3 → 2) line, which is the dominant component of the molecular gas and the one with the highest signal-to-noise ratio.

Zoom In Zoom Out Reset image size

The first moment image was created including all the CO(J = 3 → 2) emission in the velocity range −100 ≲ v_offset (km s⁻¹) ≲ +100 and masking pixels with emission below 3 times the rms noise level of the channel maps. The velocity field exhibits a clear velocity gradient along the NE–SW direction from v_offset = +80 km s⁻¹ to v_offset = −80 km s⁻¹, resembling the typical velocity field of radially expanding disks and/or tori inclined with respect to the line of sight. However, the emission with velocity offset 80 ≲ ∣v_offset(km s⁻¹)∣ ≲ 100 appears located closer to the central regions and does not show a clear velocity gradient. For the other molecules the velocity gradient also becomes unclear for velocity offsets ∣v_offset∣>80 km s⁻¹. This is probably due to the molecular gas moving in a more complicated way than the observed velocity gradient. In order to compare the ALMA observations with the spatio-kinematical model of a radially expanding disk, we used the software SHAPE (Steffen et al. 2011) to visualize the resulting velocity field. The parameters of the model were obtained as follows. Assuming that the molecular gas has expanded with constant velocity since it was ejected, the inclination of the polar axis of the disk with respect to the line of sight, i, ⁸ can be estimated from the following expression:

$$\begin{eqnarray*}&&\sin i=0.843\left[\displaystyle \frac{{{\rm{v}}}_{\mathrm{edge}}}{80\,\mathrm{km}\,{{\rm{s}}}^{-1}}\right]\left[\displaystyle \frac{\tau }{100\,\mathrm{yr}}\right]{\left[\displaystyle \frac{D}{4\,\mathrm{kpc}}\right]}^{-1}{\left[\displaystyle \frac{{\theta }_{\mathrm{maj}}}{1^{\prime\prime} }\right]}^{-1},\end{eqnarray*}$$

where v_edge is the maximum observed value of the velocity offset at the edge of the disk, τ is the time since the ejection of the molecular gas, which is assumed to have occurred at the same time as the VLTP, D is the distance to V 605 Aql, and θ_maj is the angular size of the major axis of the disk. The value of v_edge is obtained from the velocity offset at the edge of the disk in the first moment image, namely, v_edge = 80 km s⁻¹. The angular size of the major axis of the disk is obtained from an elliptical fit to the CO(J = 3 → 2) emission, which gives θ_maj ≈ 1'', with a P.A. = −37°. This orientation is in excellent agreement with the direction of a dark band in the ejecta of V 605 Aql seen at optical wavelengths (see e.g., Hinkle et al. 2008; Clayton et al. 2013). Thus, for a distance to the source of 4 kpc, the derived inclination of the polar axis of the disk is i ≈ 60°. Consequently, the de-projected expansion velocity of the disk is ${v}_{\exp }$ ≈ 90 km s⁻¹. The right panel of Figure 5 shows a spatio-kinematical model created with SHAPE, which includes a radially expanding disk inclined with respect to the line of sight, i = 60°, and with an expansion velocity ${v}_{\exp }$ = 90 km s⁻¹. It is worth noting that the SHAPE modeling does not consider radiative transfer parameters and the output just shows the spatio-kinematical characteristics of the model, which are qualitatively in good agreement with the observations.

In addition to the gradient discussed above, we found that the CO(J = 3 → 2) emission with velocity offsets −120 ≲ v_offset(km s⁻¹) ≲ −100 and +100 ≲ v_offset(km s⁻¹) ≲ +140 exhibits an inverted velocity gradient, i.e., the blue- and redshifted emission is located toward NE and SW from the center of the system, respectively (see left panel of Figure 5). Since these components are the ones with the highest velocity offset, we refer to them as high-velocity components (HVCs). HVCs, arising from a bipolar outflow, are not clearly identified or isolated as such in the emission of the other molecules. Although the HCN line also shows broad emission wings, the lack of a clear velocity gradient suggests gas moving in a more complex way than a bipolar outflow. The de-projected expansion velocity of the HVC, considering the inclination i = 60° derived above, is ≈280 km s⁻¹. The velocity gradient of the HVC is the one expected for a bipolar outflow oriented perpendicularly to the expanding disk. This is shown in the SHAPE model of Figure 5 as two lobes along the polar axis of the disk. It should be noted that the spatio-kinematical components revealed by our ALMA observations are in complete agreement with the spatial configuration derived from optical observations of the bipolar lobes of V 605 Aql, which show that emission from the SW lobe exhibits a higher degree of absorption and is redshifted, indicating that it lies behind an absorbing toroidal component.

A remarkable result from the ALMA observations is that, even though the HVC has the highest velocity offset, it lies just within ≈02 (≲1000 au) from the center of the disk. Considering that the gas is moving at ∼280 km s⁻¹, this implies a kinematical age for the bipolar outflow of ≲20 yr. This result would suggest that molecular material is currently being ejected from V 605 Aql, and that we are witnessing the collimation of mass loss by a born-again star even after 100 yr of experiencing a VLTP. An alternative explanation for the bipolar outflow is that it consists of molecular material that is being dragged by a stellar wind from the inner regions of the disk.

In order to derive the energetics of the ejected material we corrected for the inclination and assuming optically thin emission for the CO(J = 3 → 2) line and an excitation temperature T_ex = 75 K, we estimate that the scalar moment carried by the CO gas is 8.0 ± 0.1 × 10³⁶ gm cm s⁻¹ and the kinetic energy is 5.0 ± 0.1 × 10⁴³ erg. For comparison, we note that the kinetic energy of the hydrogen-deficient ejecta in other born-again PNe such as A 30, A 78, and HuBi 1 is ∼10⁴⁴–10⁴⁵ erg.

The ¹²C/¹³C abundance ratio can be estimated from the H¹²CN (J = 4 → 3) and H¹³CN (J = 4 → 3) lines, assuming that the emission is optically thin for both lines. Under this assumption, we obtain a H¹²CN/H¹³CN abundance ratio of 5.6 ± 0.6, which is essentially independent of the excitation temperature. This value is in agreement with the lower limit, ¹²C/¹³C > 4, derived by Tafoya et al. (2017) from APEX observations.

The H¹²CN/H¹³CN abundance ratio from our ALMA observations is consistent with the ¹²C/¹³C abundance ratio value of ≈5.3 obtained from VLTP calculations presented in Miller Bertolami et al. (2006), which suggest that the hydrogen-deficient ejection of material in V 605 Aql was produced by the evolution of a single star, instead of an interacting binary system in a nova-like channel as proposed by Lau et al. (2011). However, if the HVC is indeed due to a collimated outflow, the presence of a binary companion could explain the launching of such an outflow.

A possible scenario that could explain our ALMA observations is that when V 605 Aql underwent the VLTP, which caused the expansion of its external layers, it experienced a common envelope event with a companion, such as the ones discussed by Ivanova et al. (2013). This would explain the formation of the toroidal component of the hydrogen-deficient material, i.e., the expanding disk, and the presence of the ongoing bipolar ejection, i.e., the HVC. A similar scenario has been hinted at by Toalá et al. (2021a) for the case of A 30 in which a binary companion has been suggested to exist (Jacoby et al. 2020).