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Register a new userThe candidate luminous blue variable G79.29+0.46: a comprehensive study of its ejecta through a multiwavelength analysis
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We present a multiwavelength analysis of the nebula around the candidate luminous blue variable G79.29+0.46. The study is based on our radio observations performed at the Expanded Very Large Array and at the Green Bank Telescope and on archival infrared datasets, including recent images obtained by the Herschel Space Observatory. We confirm that the radio central object is characterized by a stellar wind and derive a current mass-loss rate of about 1.4x10-6 Msun yr-1. We find the presence of a dusty compact envelope close to the star, with a temperature between 40 and 1200 K. We estimate for the outer ejecta an ionised gas mass of 1.51 Msun and a warm (60--85 K) dust mass of 0.02 Msun. Diagnostics of the far-infrared spectra indicate the presence of a photo-dissociation region around the ionised gas. Finally, we model the nebula with the photo-ionization code CLOUDY, using as input parameters those estimated from our analysis. We find for the central star a luminosity of 10^5.4 Lsun and an effective temperature of 20.4 kK.
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We report the discovery of a new Galactic candidate Luminous Blue Variable (cLBV) via detection of an infrared circular nebula and follow-up spectroscopy of its central star. The nebula, MN112, is one of many dozens of circular nebulae detected at $24 mu$m in the {it Spitzer Space Telescope} archival data, whose morphology is similar to that of nebulae associated with known (c)LBVs and related evolved massive stars. Specifically, the core-halo morphology of MN112 bears a striking resemblance to the circumstellar nebula associated with the Galactic cLBV GAL 079.29+00.46, which suggests that both nebulae might have a similar origin and that the central star of MN112 is a LBV. The spectroscopy of the central star showed that its spectrum is almost identical to that of the bona fide LBV P Cygni, which also supports the LBV classification of the object. To further constrain the nature of MN112, we searched for signatures of possible high-amplitude ($ga 1$ mag) photometric variability of the central star using archival and newly obtained photometric data covering a 45 year period. We found that the B magnitude of the star was constant ($simeq$ 17.1$pm$0.3 mag) over this period, while in the I band the star brightened by $simeq 0.4$ mag during the last 17 years. Although the non-detection of large photometric variability leads us to use the prefix `candidate in the classification of MN112, we remind that the long-term photometric stability is not unusual for genuine LBVs and that the brightness of P Cygni remains relatively stable during the last three centuries.
We have observed the radio nebula surrounding the Galactic LBV candidate G79.29+0.46 with the EVLA at 6 cm. These new radio observations allow a morphological comparison between the radio emission, which traces the ionized gas component, and the mid-IR emission, a tracer of the dust component. The IRAC (8 mu m) and MIPS (24 mu m and 70 mu m) images have been reprocessed and compared with the EVLA map. We confirm the presence of a second shell at 24 mu m and also provide evidence for its detection at 70 mu m. The differences between the spatial morphology of the radio and mid-IR maps indicate the existence of two dust populations, the cooler one emitting mostly at longer wavelengths. Analysis of the two dusty, nested shells have provided us with an estimate of the characteristic timescales for shell ejection, providing important constraints for stellar evolutionary models. Finer details of the ionized gas distribution can be appreciated thanks to the improved quality of the new 6 cm image, most notably the highly structured texture of the nebula. Evidence of interaction between the nebula and the surrounding interstellar medium can be seen in the radio map, including brighter features that delineate regions where the shell structure is locally modified. In particular, the brighter filaments in the south-west region appear to frame the shocked southwestern clump reported from CO observations.
MWC 930 is a star just ~2{deg} above the Galactic plane whose nature is not clear and that has not been studied in detail so far. While a post-Asymptotic Giant Branch (AGB) classification was proposed in the past, studies of its optical spectrum and photometry pointed toward strong variability, therefore the object was reclassified as a Luminous Blue Variable (LBV) candidate. LBVs typically undergo phases of strong mass loss in the form of eruptions that can create shells of ejecta around the star. Our goal is to search for the presence of such a circumstellar nebula in MWC 930 and investigate its properties. To do so, we make use of space-based infrared data from our Spitzer campaign performed with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS) as well as data from optical and infrared (IR) surveys. In our Spitzer images, we clearly detect an extended shell around MWC 930 at wavelengths longer than 5 um. The mid-infrared spectrum is dominated by the central star and mostly shows forbidden lines of [FeII], with an underlying continuum that decreases with wavelength up to ~15 um and then inverts its slope, displaying a second peak around 60 um, evidence for cold dust grains formed in a past eruption. By modeling the SED, we identify two central components, besides the star and the outer shell. These extra sources of radiation are interpreted as material close to the central star, maybe due to a recent ejection. Features of C-bearing molecules or grains are not detected.
We present the results of long-term spectropolarimetric and spectroscopic monitoring of MWC 314, a candidate Luminous Blue Variable star. We detect the first evidence of H$alpha$ variability in MWC 314, and find no apparent periodicity in this emission. The total R-band polarization is observed to vary between 2.21% and 3.00% at a position angle consistently around $sim0^{circ}$, indicating the presence of a time-variable intrinsic polarization component, hence an asymmetrical circumstellar envelope. We find suggestive evidence that MWC 314s intrinsic polarization exhibits a wavelength-independent magnitude varying between 0.09% and 0.58% at a wavelength-independent position angle covering all four quadrants of the Stokes Q-U plane. Electron scattering off of density clumps in MWC 314s wind is considered as the probable mechanism responsible for these variations.
We present the results of study of the Galactic candidate luminous blue variable Wray 15-906, revealed via detection of its infrared circumstellar shell (of approx2 pc in diameter) with the Wide-field Infrared Survey Explorer and the Herschel Space Observatory. Using the stellar atmosphere code CMFGEN and the Gaia parallax, we found that Wray 15-906 is a relatively low-luminosity, log(L/Lsun)approx5.4, star of temperature of 25pm2 kK, with a mass-loss rate of approx3times10^{-5} Msun/yr, a wind velocity of 280pm50 km/s, and a surface helium abundance of 65pm2 per cent (by mass). In the framework of single star evolution, the obtained results suggest that Wray 15-906 is a post-red supergiant star with initial mass of approx26pm2 Msun and that before exploding as a supernova it could transform for a short time into a WN11h star. Our spectroscopic monitoring with the Southern African Large Telescope (SALT) does not reveal significant changes in the spectrum of Wray 15-906 during the last 8 yr, while the V-band light curve of this star over years 1999--2019 shows quasi-periodic variability with a period of approx1700 d and an amplitude of approx0.1 mag. We estimated the mass of the shell to be 2.9pm0.5 Msun assuming the gas-to-dust mass ratio of 200. The presence of such a shell indicates that Wray 15-906 has suffered substantial mass loss in the recent past. We found that the open star cluster C1128-631 could be the birth place of Wray 15-906 provided that this star is a rejuvenated product of binary evolution (a blue straggler).
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