Millimetre continuum from LBV stars and their environs. NIKA2 observations and Virtual Observatory data

TL;DR

This study uses NIKA2 millimetre continuum observations, complemented by Virtual Observatory data, to dissect the emission around five Galactic LBVs. By constructing six-decade SEDs and applying a minimal-parameter multi-component model, the authors separate contributions from stellar photospheres, circumstellar dust, thermal winds, and ionised gas. They find unresolved hot dust very close to the stars in most cases and derive first fundamental parameters for MN101, while revealing two distinct SED families linked to effective temperature. The work demonstrates that the millimetre window bridges far-IR and radio regimes, providing critical constraints on dust and free-free emission and highlighting the mm-band as a powerful probe of mass and energy feedback from LBVs to the interstellar medium.

Abstract

Multi-wavelength studies of their circumstellar environments around LBVs are essential to quantify their feedback at Galactic scales. Dominant emission mechanisms at millimetre wavelengths are, however, still poorly understood. Stellar winds, circumstellar dust, and ionised gas have not been explored together in the case of LBVs. We aim to study the millimetre continuum emission of Galactic LBVs to disclose the presence of these components, to describe their morphology and to measure their relevance in the mass and energy injection to the interstellar medium. We have used the NIKA2 continuum camera at the IRAM 30-m radio telescope to observe and analyse 1.15 and 2 mm continuum from the LBVs HD168607, HD168625, MN87, MN101, and G79.29+0.46. We used Virtual Observatory to complement our observations with archival data from optical, infrared, mm- and cm-wavelengths. With this information, we have built complete SEDs for the five sources, covering six decades of the electromagnetic spectrum. All targets except MN87 were detected at both wavelengths, with features including compact sources, extended nebular emission, shells, and unrelated background structures. Spectral indices of compact sources are consistent with thermal emission from stellar winds. We modelled the SEDs and successfully reproduced the emission from stellar photospheres, circumstellar dust, thermal stellar winds and enshrouding HII regions. Our models, in agreement with previous literature results, reveal the presence of unresolved hot dust very close to the stars and provide the first estimates for the fundamental parameters of MN101. This pilot study highlights the great potential of millimetre continuum studies of LBVs and possibly other evolved massive stars. The mm spectral window bridges the far-IR and radio regimes and can disclose the relative contribution of dust and free-free emission in this kind of sources.

Figures (11)

• Figure 1: Noise map of the fields observed. The $rms$ noise was computed in concentric rings of 2 arcsec wide. The noise in the 1.15 mm maps are plotted in blue, while the one corresponding to the 2 mm maps are in red. Field names (abridged) are indicated near the 1 mm curves.
• Figure 2: Sky maps of the mm-continuum emission in the four fields observed with NIKA2. Maps at 1.15 mm are at the top row, while the 2 mm maps are on the bottom one. The field which contains HD168607 and HD168625, labelled HD168*, is centered in HD168625. Colour scales --indicated or the right of each row-- are in Jy beam$^{-1}$. HPBWs are drawn near the bottom left corners of the HD168* images. Central areas indicate the zoomed region displayed in Fig. \ref{['fig:zoom']}.
• Figure 3: Zoomed sky maps around the central sources. The regions correspond to those indicated in Fig. \ref{['fig:fields']}. To span the full dynamic range, colour scale is different for each of the maps. Contours correspond to 30, 50 (highlighted in white), and 70 % of the maximum value. HPBWs are indicated near the bottom of the HD168* images. The contours at 50 % are highlighted to facilitate a visual comparison with the beam, and give an idea about the point-like nature of the central sources.
• Figure 4: Spectral index maps between the two NIKA2 frequencies. The plotted field and marks are the same as in Fig. \ref{['fig:fields']}. For each map, all pixels with brightness below 2$\sigma$ have been masked.
• Figure 5: Spectral energy distribution of HD168607, in logarithmic scale. Catalogues are indicated on top right. All points have their corresponding error bars plotted. The NIKA2 points are shown with blue stars.