CO, Water, and Possible Methanol in Eta 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 $\eta$ 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 ($T_{\mathrm{gas}}\simeq$ 700 K) material, $\sim$2${}^{\prime \prime }$ (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 ($T_{\mathrm{gas}}\simeq$ 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 $\eta$ 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.

arXiv commentAccepted by ApJ Letters

References in corpus (49)

• The Herschel-SPIRE instrument and its in-flight performance
• The CO-to-H2 Conversion Factor
• Binary interaction dominates the evolution of massive stars
• A computer program for fast non-LTE analysis of interstellar line spectra
• Mind your Ps and Qs: the Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars
• Observations of the Icy Universe
• The incidence of stellar mergers and mass gainers among massive stars
• The Structure of the Homunculus: I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of Eta Carinae
• In Hot Pursuit of the Hidden Companion of Eta Carinae: An X-ray Determination of the Wind Parameters
• Luminous Blue Variables and superluminous supernovae from binary mergers
• Water Abundance in Molecular Cloud Cores
• The periodicity of the eta Carinae events
• Structure and evolution of high-mass stellar mergers
• A Revised Historical Light Curve of Eta Carinae and the Timing of Close Periastron Encounters
• Herschel observations of EXtraordinary Sources: Analysis of the full Herschel/HIFI molecular line survey of Sagittarius B2(N)
• High-excitation emission lines near eta Carinae, and its likely companion star
• Constraining the Absolute Orientation of Eta Carinae's Binary Orbit: A 3-D Dynamical Model for the Broad [Fe III] Emission
• Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations
• Constraints on decreases in Eta Carinae's mass loss from 3D hydrodynamic simulations of its binary colliding winds
• A multispectral view of the periodic events in eta Carinae
• Light-echoes from the plateau in Eta Carinae's Great Eruption reveal a two-stage shock-powered event
• On the influence of the companion star in Eta Carinae: 2D radiative transfer modeling of the ultraviolet and optical spectra
• Was the nineteenth century giant eruption of Eta Carinae a merger event in a triple system?
• The massive protostar W43-MM1 as seen by Herschel-HIFI water spectra: high turbulence and accretion luminosity
• Non-thermal X-rays from Colliding Wind Shock Acceleration in the Massive Binary Eta Carinae
• Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores
• He II $\lambda$4686 emission from the massive binary system in $\eta$ Car: constraints to the orbital elements and the nature of the periodic minima
• The Abundance, Ortho/Para Ratio, and Deuteration of Water in the High-Mass Star Forming Region NGC 6334 I
• $\eta$ Carinae's Dusty Homunculus Nebula from Near-Infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity
• The Systemic Velocity of Eta Carinae
• An investigation of the molecular clouds of the Carina HII region/molecular cloud complex - First results
• The three-dimensional structure of the Eta Carinae Homunculus
• Understanding the X-ray Flaring from Eta Car
• The Water Vapor Abundance in Orion KL Outflows
• The fossil wind structures of Eta Carinae: changes across on 5.54-year cycle
• Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH231.8+4.2
• A lambda 3mm and 1mm line survey toward the yellow hypergiant IRC +10420
• A Lighthouse Effect in Eta Carinae
• Distinguishing Circumstellar from Stellar Photometric Variability in Eta Carinae
• $\eta$ Carinae Baby Homunculus Uncovered by ALMA
• A disrupted molecular torus around Eta Carinae as seen in 12CO with ALMA
• Molecules in η Carinae
• HD101584: Circumstellar characteristics and evolutionary status
• Mid-infrared evolution of eta Car from 1968 to 2018
• First detection of methanol towards a post-AGB object, HD101584
• The peculiar chemistry of the inner ejecta of Eta Carina
• Towards detecting methanol emission in low-mass protoplanetary discs with ALMA: The role of non-LTE excitation
• Spatial distribution and kinematics of the molecular material associated with $\eta$ Carinae
• Inverted Ionization Zones in Eta Carinae's "Weigelt Knots"

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• The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. II. Kinematic structure
• A warm molecular ring in AG Car: composing the mass-loss puzzle
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