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Register a new userA slowly expanding torus associated with the candidate LBV MGE 042.0787+00.5084
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The luminous blue variable (LBV) phase is a poorly understood stage in the evolution of high mass stars, characterized for its brevity and instability. The surroundings of LBV stars are excellent test beds to study their dense stellar winds and eruptive mass-loss events. Aiming to improve our knowledge of the LBV phase, we observed the J=1-0 and J=2-1 lines of CO and $^{13}$CO in a field of 1.5x1.5 around the recently identified candidate LBV MGE 042.0787+00.5084, using the IRAM 30-m radio telescope. We report the first detection of molecular emission associated with this source, tracing a structure with an evident circumstellar distribution. Morphology and kinematics of the gas can be explained by an expanding torus, a structure that may have originated from stellar ejecta or the action of stellar winds onto the parent molecular cloud. We derive the physical properties of the gas by means of LTE and non-LTE line modelling, obtaining densities of H$_2$ in the order of 10$^3$ cm$^{-3}$ and kinetic temperatures below 100 K. In addition, we build a kinematic model to reproduce the structure and observed velocity fields of the gas, which is in good agreement with the observations. We estimate a total molecular gas mass of 0.6$pm$0.1 Msun and a dynamical age of 6$times$10$^4$ years, leading to an average mass-loss rate of 0.8-1.2$times$10$^{-5}$ Msun yr$^{-1}$.
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Only about 19 Galactic and 25 extra-galactic bona-fide Luminous Blue Variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and H{alpha} luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have M(V) less than or equal to -7 and a combination of: H{alpha} with a P-Cygni profile; no [O I] 6300 emission; weak or no [O III] 5007 emission; large [N II]/H{alpha} relative to H II regions; and [S II] 6716 / [S II] 6731 similar to 1. It is not clear if these five cLBVs are isolated from O-type stars, which would favor the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies.
We present new observations of the nebula around the Magellanic candidate Luminous Blue Variable S61. These comprise high-resolution data acquired with the Australia Telescope Compact Array (ATCA), the Atacama Large Millimetre/Submillimetre Array (ALMA), and VISIR at the Very Large Telescope (VLT). The nebula was detected only in the radio, up to 17 GHz. The 17 GHz ATCA map, with 0.8 arcsec resolution, allowed a morphological comparison with the H$alpha$ Hubble Space Telescope image. The radio nebula resembles a spherical shell, as in the optical. The spectral index map indicates that the radio emission is due to free-free transitions in the ionised, optically thin gas, but there are hints of inhomogeneities. We present our new public code RHOCUBE to model 3D density distributions, and determine via Bayesian inference the nebulas geometric parameters. We applied the code to model the electron density distribution in the S61 nebula. We found that different distributions fit the data, but all of them converge to the same ionised mass, ~0.1 $rm Modot$, which is an order of magnitude smaller than previous estimates. We show how the nebula models can be used to derive the mass-loss history with high-temporal resolution. The nebula was probably formed through stellar winds, rather than eruptions. From the ALMA and VISIR non-detections, plus the derived extinction map, we deduce that the infrared emission observed by space telescopes must arise from extended, diffuse dust within the ionised region.
We present a comprehensive analysis of 20 years worth of multi-color photometric light curves, multi-epoch optical spectra, and X-ray data of an off-nuclear variable object SDSS1133 in Mrk 177 at $z=0.0079$. The UV-optical light curves reveal that SDSS1133 experienced three outbursts in 2001, 2014, and 2019. The persistent UV-optical luminosity in the non-outbursting state is $sim 10^{41}$ erg/s with small-scale flux variations, and peak luminosities during the outbursts reach $sim 10^{42}$ erg/s. The optical spectra exhibit enduring broad hydrogen Balmer P-Cygni profiles with the absorption minimum at $sim -2,000$ km/s, indicating the presence of fast moving ejecta. Chandra detected weak X-ray emission at a 0.3-10 keV luminosity of $L_{X} = 4 times 10^{38}$ erg/s after the 2019 outburst. These lines of evidence strongly suggests that SDSS1133 is an extremely luminous blue variable (LBV) star experiencing multiple giant eruptions with interactions of the ejected shell with different shells and/or circumstellar medium (CSM), and strongly disfavors the recoiling Active Galactic Nuclei (AGN) scenario suggested in the literature. We suggest that pulsational pair-instability may provide a viable explanation for the multiple energetic eruptions in SDSS1133. If the current activity of SDSS1133 is a precursor of a supernova explosion, we may be able to observe a few additional giant eruptions and then the terminal supernova explosion in future observations.
We have mapped 12CO J=3-2 and other molecular lines from the water-fountain bipolar pre-planetary nebula (PPN) IRAS 16342-3814 with ~0.35 resolution using ALMA. We find (i) two very high-speed knotty, jet-like molecular outflows, (ii) a central high-density (> few x 10^6 cm^{-3}), expanding torus of diameter 1300 AU, and (iii) the circumstellar envelope of the progenitor AGB, generated by a sudden, very large increase in the mass-loss rate to >3.5 x 10^{-4} Msun/yr in the past ~455 yr. Strong continuum emission at 0.89 mm from a central source (690 mJy), if due to thermally-emitting dust, implies a substantial mass (0.017 Msun) of very large (~mm-sized) grains. The measured expansion ages of the above structural components imply that the torus (age~160 yr) and the younger high-velocity outflow (age~110 yr) were formed soon after the sharp increase in the AGB mass-loss rate. Assuming a binary model for the jets in IRAS 16342, the high momentum rate for the dominant jet-outflow in IRAS 16342 implies a high minimum accretion rate, ruling out standard Bondi-Hoyle-Lyttleton wind accretion and wind Roche lobe overflow (RLOF) models with white-dwarf or main-sequence companions. Most likely, enhanced RLOF from the primary or accretion modes operating within common envelope evolution are needed.
$eta$~Car is one of the most massive stars in the Galaxy. It underwent a massive eruption in the 19th century, which produced the impressive bipolar Homunculus nebula now surrounding it. The central star is an eccentric binary with a period of 5.54,years. Although the companion has not been detected directly, it causes time-variable ionization and colliding-wind X-ray emission. By characterizing the complex structure and kinematics of the ejecta close to the star, we aim to constrain past and present mass loss of $eta$~Car. $eta$~Car is observed with the extreme adaptive optics instrument SPHERE at the Very Large Telescope, using its polarimetric mode in the optical with the ZIMPOL camera. A spatial resolution of 20,mas was achieved, i.e. very close to the presumed 13 mas apastron separation of the companion star. We detect new structures within the inner arcsecond to the star (2,300,au at a 2.3,kpc distance). We can relate these structures to the eruption near 1890 by tracking their proper motions derived from our new images and historical images over a 30,years time span. Besides, we find a fan-shaped structure in the inner 200~au to the star in the H$alpha$ line, that could potentially be associated with the wind collision zone of the two stars.
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