The Gaia-ESO Survey: dynamics of ionized and neutral gas in the Lagoon nebula (M8)

Abstract

We present a spectroscopic study of the dynamics of the ionized and neutral gas throughout the Lagoon nebula (M8), using VLT/FLAMES data from the Gaia-ESO Survey. We explore the connections between the nebular gas and the stellar population of the associated star cluster NGC6530. We characterize through spectral fitting emission lines of H-alpha, [N II] and [S II] doublets, [O III], and absorption lines of sodium D doublet, using data from the FLAMES/Giraffe and UVES spectrographs, on more than 1000 sightlines towards the entire face of the Lagoon nebula. Gas temperatures are derived from line-width comparisons, densities from the [S II] doublet ratio, and ionization parameter from H-alpha/[N II] ratio. Although doubly-peaked emission profiles are rarely found, line asymmetries often imply multiple velocity components along the line of sight. This is especially true for the sodium absorption, and for the [O III] lines. Spatial maps for density and ionization are derived, and compared to other known properties of the nebula and of its massive stars 9 Sgr, Herschel 36 and HD 165052 which are confirmed to provide most of the ionizing flux. The detailed velocity fields across the nebula show several expanding shells, related to the cluster NGC6530, the O stars 9 Sgr and Herschel 36, and the massive protostar M8East-IR. The origins of kinematical expansion and ionization of the NGC6530 shell appear to be different. We are able to put constrains on the line-of-sight (relative or absolute) distances between some of these objects and the molecular cloud. The large obscuring band running through the middle of the nebula is being compressed by both sides, which might explain its enhanced density. We also find an unexplained large-scale velocity gradient across the entire nebula. At larger distances, the transition from ionized to neutral gas is studied using the sodium lines.

26 pages, 31 figures, accepted on Astronomy and Astrophysics journal

References in corpus (4)

• The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+)
• Spectroscopic analysis of southern B and Be stars
• The S2N2 metallicity calibrator and the abundance gradient of M 33
• VLT/Flames observations of the star forming region NGC 6530

Cited by in corpus (12)

• The Gaia-ESO Public Spectroscopic Survey: Implementation, data products, open cluster survey, science, and legacy
• OB associations and their origins
• Gradients of chemical abundances in the Milky Way from HII regions: distances derived from Gaia EDR3 parallaxes and temperature inhomogeneities
• The Gaia-ESO Survey: Asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2
• The Gaia-ESO Survey: age spread in the star forming region NGC6530 from the HR diagram and gravity indicators
• A wide-area photometric and astrometric (Gaia DR2) study of the young cluster NGC 6530
• The GALAH survey: Characterization of emission-line stars with spectral modelling using autoencoders
• Observational study of hydrocarbons in the bright photodissociation region of Messier 8
• Turbulence in compact to giant H II regions
• Near-Infrared Diffuse Interstellar Bands towards Her 36
• The internal line-of-sight kinematics of NGC 346: the rotation of the core region
• SDSS-V LVM: A spatially resolved study of the physical conditions and the chemical abundance discrepancy in the Lagoon Nebula (M 8)