Showing papers by "Alex Lobel published in 2015"
TL;DR: In this paper, the authors used two-colour diagrams based on atmosphere models, spectral energy distributions, and different extinctions and extinction laws, depending on the location of the supposed BL source: either in Gum48d on the background or in the envelope of the yellow hypergiant HR 5171A.
Abstract: Aims. We re-examined photometry (VBLUW , UBV , uvby ) of the yellow hypergiant HR 5171A made a few decades ago. In that study no proper explanation could be given for the enigmatic brightness excesses in the L band (VBLUW system, λ eff = 3838 A). In the present paper, we suggest that this might have been caused by blue luminescence (BL), an emission feature of neutral polycyclic aromatic hydrocarbon molecules (PAHs), discovered in 2004. It is a fact that the highest emission peaks of the BL lie in the L band. Our goals were to investigate other possible causes, and to derive the fluxes of the emission.Methods. We used two-colour diagrams based on atmosphere models, spectral energy distributions, and different extinctions and extinction laws, depending on the location of the supposed BL source: either in Gum48d on the background or in the envelope of HR 5171A.Results. False L –excess sources, such as a hot companion, a nearby star, or some instrumental effect, could be excluded. Also, emission features from a hot chromosphere are not plausible. The fluxes of the L excess, recorded in the data sets of 1971, 1973, and 1977 varied (all in units of 10-10 Wm-2 μ m-1 ) between 1.4 to 21, depending on the location of the source. A flux near the low side of this range is preferred. Small brightness excesses in uv (uvby system) were present in 1979, but its connection with BL is doubtful. For the L fluxes we consider the lowest values as more realistic. The uncertainties are 20–30 %. Similar to other yellow hypergiants, HR 5171A showed powerful brightness outbursts, particularly in the 1970s. A release of stored H-ionization energy by atmospheric instabilities could create BL emitted by neutral PAHs.
9 citations
TL;DR: In this article, a detailed study of the detailed spectroscopic variability of selected Fe-i absorption lines observed between 1991 and 1995 in the optical spectra of the bright yellow hypergiants HR 5171A and Rho Cas is presented.
Abstract: We present a comparative study of the detailed spectroscopic variability of selected Fe i absorption lines observed between 1991 and 1995 in the optical spectra of the bright yellow hypergiants HR 5171A and Rho Cas. We also compare with the high-resolution spectrum of HR 8752. The three cool hypergiants reveal broad lines with very similar shapes. The variations with time in the line shapes and depths of HR 5171A and Rho Cas are very similar. The spectroscopic variability is caused by quasi-periodic atmospheric pulsations that strongly correlates with the observed radial velocity and V -brightness changes.
4 citations
TL;DR: In this paper, the authors used two-colour diagrams based on atmosphere models, spectral energy distributions, and different extinctions and extinction laws, depending on the location of the supposed blue luminescence (BL), either in Gum48d on the background or in the envelope of HR 5171A.
Abstract: We re-examined photometry (VBLUW, UBV, uvby) of the yellow hypergiant HR 5171A made a few decades ago. In that study no proper explanation could be given for the enigmatic brightness excesses in the L band (VBLUW system, lambda_eff=3838 A). In the present paper, we suggest that this might have been caused by blue luminescence (BL), an emission feature of neutral polycyclic aromatic hydrocarbon molecules (PAHs), discovered in 2004. It is a fact that the highest emission peaks of the BL lie in the L band. Our goals were to investigate other possible causes, and to derive the fluxes of the emission. We used two-colour diagrams based on atmosphere models, spectral energy distributions, and different extinctions and extinction laws, depending on the location of the supposed BL source: either in Gum48d on the background or in the envelope of HR 5171A. False L-excess sources, such as a hot companion, a nearby star, or some instrumental effect, could be excluded. Also, emission features from a hot chromosphere are not plausible. The fluxes of the L excess, recorded in the data sets of 1971, 1973, and 1977 varied (all in units of 10^(-10) W m^(-2) micron^(-1)) between 1.4 to 21, depending on the location of the source. A flux near the low side of this range is preferred. Small brightness excesses in uv (uvby system) were present in 1979, but its connection with BL is doubtful. For the L fluxes we consider the lowest values as more realistic. The uncertainties are 20-30 %. Similar to other yellow hypergiants, HR 5171A showed powerful brightness outbursts, particularly in the 1970s. A release of stored H-ionization energy by atmospheric instabilities could create BL emitted by neutral PAHs.
3 citations
TL;DR: In this paper, discrete absorption components (DACs) were observed in extended violet absorption wings of P Cyg Hei lines at orbital phases when the primary is in front of the secondary.
Abstract: VLT-NACO imaging and spectroscopic monitoring of MWC 314 shows it is a hierarchical triple system consisting of a close massive binary that contains a Luminous Blue Variable (LBV), and a third companion bound in a wide orbit. We observe Discrete Absorption Components (DACs) in extended violet absorption wings of P Cyg He i lines at orbital phases when the primary is in front of the secondary. The DACs signal fast expanding wind regions of enhanced density and variable outflow velocity. We present a model of the circumbinary disc showing that the formation region of Fe ii emission lines in the disc extends to ∼26 au from the center of gravity for i =70 ∘ . Direct imaging of the candidate LBV HD 168625 also reveals a companion bound in a wide orbit. A kinematic analysis of VLT-UVES slit spectra signals significantly different radial velocities of the [N ii] λ6584 emission line formation regions N-S across the inner loop-like nebular structures.