CO, Water, and Tentative Methanol in η Carinae Approaching Periastron

Abstract

In circumstellar gas, the complex organic molecule methanol has been found almost exclusively around young stellar objects, and is thus regarded as a signpost of recent star formation. Here we report the first probable detection of methanol around an evolved high-mass star, in the complex circumstellar environment around the Luminous Blue Variable η Carinae, while using ALMA to investigate molecular cloud conditions traced by CO (2-1) in an orbit phase of the massive binary preceding the 2020 periastron. Favoring methanol over a 13 CS alternative, the emission originates from hot (T gas 700 K) material, ∼2 (0.02 pc) across, centered on the dust-obscured binary in contrast to the CO which traces inner layers of the extended massive equatorial torus, and is accompanied by prominent absorption in a cooler (T gas 110 K) layer of gas. We also report detections of water in Herschel/HIFI observations at 557 GHz and 988 GHz. The methanol abundance is several to 50 times higher than observed towards several lower mass stars, while water abundances are similar to those observed in cool, dense molecular clouds. The very high methanol:water abundance ratio in the core of η Carinae may suggest methanol formation processes similar to Fischer-Tropsch-type catalytic reactions on dust grains. These observations prove that complex molecule formation can occur in the chemically evolved environments around massive stars in the end stages of their evolution, given sufficient gas densities and shielding conditions as may occur in material around massive interacting companions and merger remnants.

Eta carinae and the homunculus: far infrared/submillimetre spectral lines detected with the Herschel Space Observatory

“…After H2, OH and CH are the first molecules detected in η Car, through their UV electronic transitions and moving in gas at −513 km s −1 , using the HST/STIS (Verner et al 2005b). Detection of the rotational lines is a significant confirmation of this key participant in the creation and regulation of water, which has been reported in Herschel/HIFI observations by Morris et al (2020), and may represent a primary sink of free oxygen in reaction with H2, i.e., O + H2 → OH + H. In addition to the NH transitions which are discussed below, two weak line complexes of NH2 (152.80 and 159.75 µm) and one weak line complex of NH3 (127.11µm) are detected (see Table 2). Both molecules have ortho and para spin symmetries whose relative abundances are sensitive to the ortho-to-para ratio of H2.…”

Spectroscopic signatures of the vanishing natural coronagraph of Eta Carinae

“…Although ALMA mapped molecular absorption lines, they should trace the warm/cold dust. Bordiu & Rizzo (2019) found CO (−9 km s −1 ), HCN (Hydrogen cyanide), and H CN (−60 km s −1 ) in absorption in front of the star at T ∼ 500 K. Morris et al (2020) found an absorption spot they attributed to CH 3 OH (Bordiu & Rizzo 2019) and another of 12 CO at radial velocity +3 ± 5 km s −1 in front of the star at T ∼ 100-110 K. Different velocities and temperatures suggest that the absorbing material (inside the occulter) are spread at different radii in front of the star. These molecular absorption lines should be fading, but only future visits by ALMA can test this hypothesis.…”

Molecular remnant of Nova 1670 (CK Vulpeculae)

“…Also in this object methanol is found in bipolar lobes and is interpreted as a signature of circumstellar shocks. Most recently, methanol was tentatively found by Morris et al (2020) in the massive interacting binary η Car, which has a nitrogen-rich molecular environment (Loinard et al 2012), has undergone an eruption nearly two centuries ago, and has circumstellar material which has been repeatedly shocked in binary wind interactions. In the case of η Car, the detection was made in an absorption spectrum so that the overall distribution of methanol in the envelope is yet unknown.…”