CO in the C1 globule of the Helix nebula with ALMA

Abstract

We present and analyse 12 CO, 13 CO and C 18 O(2-1) ALMA observations of the C1 globule inside the Helix nebula in order to determine its physical properties. Our findings confirm the molecular nature of the globule with a multi-peak structure. The 12 CO line has a high optical depth τ ∼10. The derived 12 C/ 13 C∼10 and 16 O/ 18 O∼115 ratios are not in agreement with the expected isotopic ratios of carbon-rich AGB stars. Assuming that the 12 CO optical depth has been underestimated, we can find a consistent fit for an initial mass of 2 M . We obtain a molecular mass of ∼2 ×10 −4 M for the C1 globule, which is much higher than its mass in the literature. Clumping could play a role in the high molecular mass of the knot. The origin of the tail is discussed. Our findings show that the most probable model appears to be shadowing. The kinematics and molecular morphology of the knot are not consistent with a wind-swept model and the photoevaporation model alone is not enough to explain the nature of the globule.We propose an integrated model where the effects of the photoevaporation, the stream and shadowing models are all considered in the tail shaping process.

H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543

“…Brγ emission is not detected in this knot and a lower limit is provided for the R(Brγ), assuming as upper limit intensity that of the marginally detected H 2 2-1 S(2) line at 2.154 µm (see Table 2 in Matsuura et al 2007). The H 2 excitation diagram of the K1 Helix's knot indicate a thermal origin for its H 2 emission and the dominant mechanism for the excitation of molecular hydrogen is UV-fluorescence pumping, rather than shocks (O'Dell et al 2000;Matsuura et al 2007;Andriantsaralaza et al 2020). We also included measurements made for a diffuse region and a knot in Dumbbell by Baldridge (2017).…”

Sampling Molecular Gas in the Helix Planetary Nebula: Variation in HNC/HCN with UV Flux

“…Preliminary analysis of ALMA interferometric mapping of Globule B (Bublitz et al 2021, in prep. ) indicates that its structure is similar to that of Globule C observed with ALMA (Andriantsaralaza et al 2019). Thus, the structure of Globule B is represented by an elongated rectangle of dimensions 12 by 2 pointing away from the CSPN, for the purposes of beam dilution correction.…”

“…The mapping might then be identifying the lack of molecular CO in the ionized shell that protects the cold molecule-rich gas from the CSPN's radiation. Newly published ALMA maps (J = 2 → 1 transitions of 12 CO, C 18 O, and 13 CO) have revealed that Helix Globule C consists of a N-S-oriented filamentary structure with a length of 11-16 and average thickness of 2 (Andriantsaralaza et al 2019). The ALMA data yield a velocity-and spatially-integrated line flux of 14.4 Jy km s −1 .…”

“…A characteristic mass of ∼10 −5 M has been attributed to individuals in the globule population (Meaburn et al 1996;O'Dell et al 2007), with a typical density of 10 6 cm −3 (Huggins et al 1992;Meaburn et al 1996). A recent analysis of one such globule (identified as C1, Huggins et al 1992) uses ALMA maps of CO isotopologues to refine these estimates, finding a molecular mass of ∼2×10 −4 M (Andriantsaralaza et al 2019). The internal structures of globules, such as their temperature and density gradients, have yet to be established.…”

Irradiation Investigation: Exploring the Molecular Gas in NGC 7293

“…Distinct hotspots can be seen in HCN, HCO + , and HNC, though the emission generally traces that of 12 CO. The 12 CO image of Globule C was originally proposed by P. Huggins in 2014, with analysis and publication performed contemporaneously with the new observations [18]. A detailed analysis of the imaged molecules and their analysis is forthcoming (Bublitz et al, in prep).…”