M-type Supergiant Research Articles

Evolution of the magnetic field of Betelgeuse from 2009–2017

Context. Betelgeuse is an M-type supergiant that presents a circularly polarized (Stokes V) signal in its line profiles, interpreted in terms of a surface magnetic field. Aims. The weak circular polarization signal has been monitored over 7.5 years in order to follow its evolution on different timescales, and eventually to determine its physical origin. Linear polarization measurements have also been obtained regularly in the last few years. Methods. We used both the ESPaDOnS and Narval spectropolarimeters to obtain high signal-to-noise ratio spectra, which were processed by means of the least-squares deconvolution method. In order to ensure the reality of the very weak circular polarization, special care has been taken to limit instrumental effects. In addition, several tests were performed on the Stokes V signal to establish its stellar and Zeeman origin. Results. We confirm the magnetic nature of the circular polarization, pointing to a surface magnetic field of the order of 1 G. The Stokes V profiles present variations over different timescales, the most prominent one being close to the long secondary period (LSP; around 2000 d for Betelgeuse) often invoked in red evolved stars. This long period is also dominant for all the other Stokes parameters. The circular polarization is tentatively modeled by means of magnetic field concentrations mimicking spots, showing in particular that the velocity associated with each “spot” also follows the long timescale, and that this signal is nearly always slightly redshifted. Conclusions. From the coupled variations of both linear and circular polarization signatures in amplitude, velocity and timescale, we favour giant convection cells as the main engine at the origin of polarization signatures and variations in all the Stokes parameters. This strengthens support for the hypothesis that large convective cells are at the origin of the LSP.

Measuring surface magnetic fields of red supergiant stars

Context.Red supergiant (RSG) stars are very massive cool evolved stars. Recently, a weak magnetic field was measured at the surface ofαOri and this is so far the only M-type supergiant for which a direct detection of a surface magnetic field has been reported.Aims.By extending the search for surface magnetic field in a sample of late-type supergiants, we want to determine whether the surface magnetic field detected onαOri is a common feature among the M-type supergiants.Methods.With the spectropolarimeter Narval at TélescopeBernard-Lyotwe undertook a search for surface magnetic fields in a sample of cool supergiant stars, and we analysed circular polarisation spectra using the least-squares deconvolution technique.Results.We detect weak Zeeman signatures of stellar origin in the targets CE Tau,α1Her andμCep. For the latter star, we also show that cross-talk from the strong linear polarisation signals detected on this star must be taken into account. For CE Tau andμCep, the longitudinal component of the detected surface fields is at the Gauss-level, such as inαOri. We measured a longitudinal field almost an order of magnitude stronger forα1Her. We also report variability of the longitudinal magnetic field of CE Tau andα1Her, with changes in good agreement with the typical atmospheric dynamics time-scales. We also report a non-detection of magnetic field at the surface of the yellow supergiant starρCas.Conclusions.The two RSG stars of our sample, CE Tau andμCep, display magnetic fields very similar to that ofαOri. The non-detection of a magnetic field on the post-RSG starρCas suggests that the magnetic field disappears, or at least becomes undetectable with present methods, at later evolutionary stages. Our analysis ofα1Her supports the proposed reclassification of the star as an M-type asymptotic giant branch star.

On the progenitor of the Type IIP SN 2013ej in M74

Abstract We use natural seeing imaging of SN 2013ej in M74 to identify a progenitor candidate in archival Hubble Space Telescope (HST) + Advanced Camera for Survey images. We find a source coincident with the supernova (SN) in the F814W filter within the total 75 mas (∼3 pc astrometric uncertainty; however, the position of the progenitor candidate in contemporaneous F435W and F555W filters is significantly offset. We conclude that the ‘progenitor candidate’ is in fact two physically unrelated sources; a blue source which is likely unrelated to the SN, and a red source which we suggest exploded as SN 2013ej. Deep images with the same instrument on board HST taken when the SN has faded (in approximately two year's time) will allow us to accurately characterize the unrelated neighbouring source and hence determine the intrinsic flux of the progenitor in three filters. We suggest that the F814W flux is dominated by the progenitor of SN 2013ej, and assuming a bolometric correction appropriate to an M-type supergiant, we estimate that the mass of the progenitor of SN 2013ej was between 8 and 15.5 M⊙.

A new look at the long-period eclipsing binary V383 Scorpii

V383Sco was discovered to be an eclipsing binary at the beginning of the XX century. This system has one of the longest orbital periods known (13.5yr) and was initially classified as a zet_Aur-type variable. It was then forgotten for decades. This study provides a detailed look at the V383Sco, using new data obtained around the last eclipse in 2007/8. There was a suspicion that this system could be similar to eclipsing systems with extensive dusty disks like EECep and eps_Aur. This and other, alternative hypotheses are considered. The ASAS-3 VI light curves have been used to examine photometric changes. Low-(LRS) and high-res.(HRS) spectra have been used for spectral classification, to analyse line profiles, as well as to determine the reddening, radial velocities (RVs) and distance. The SED was analysed. Using original numerical code, we performed a simplified model of the eclipse, taking into account the pulsations of one of the components. The LRS shows traces of molecular bands, characteristic of an M-type supergiant. The presence of this star in the system is confirmed by SED, by a strong dependence of the eclipse depth on the photometric bands, and by pulsational changes. The presence of a low excitation nebula around the system has been inferred from [OI] 6300A emission. Analysis of the RVs, reddening, and P-L relation for Mira-type stars imply a distance to the V383Sco of 8.4+-0.6 kpc. The distance to the nearby V381Sco is 6.4+-0.8 kpc. The very different and oppositely directed RVs of these systems (89.8 vs -178.8 km/s) seem to be in agreement with a bulge/bar kinematic model of the Galactic centre and inconsistent with purely circular motion. We have found evidence for the presence of a pulsating M-type supergiant in the V383Sco which periodically obscures the much more luminous F0I-type star, causing the deep (possibly total) eclipses which vary in duration and shape.

High-resolution optical spectroscopy of V838 Monocerotis in 2009

Context. V838 Mon erupted at the beginning of 2002. In the course of the outburst the object evolved to low effective temperatures and declined as a very late M-type supergiant. Among various scenarios proposed to explain the nature of the outburst, the most promising is a stellar merger event.

On the nature of the progenitors of three Type II-P supernovae: 2004et, 2006my and 2006ov

The pre-explosion observations of the type II-P supernovae 2006my, 2006ov and 2004et, are re-analysed. In the cases of supernovae 2006my and 2006ov we argue that the published candidate progenitors are not coincident with their respective supernova sites in pre-explosion Hubble Space Telescope observations. We therefore derive upper luminosity and mass limits for the unseen progenitors of both these supernovae, assuming they are red supergiants: 2006my (log L/Lsun = 4.51; mass < 13Msun) and 2006ov (log L/Lsun = 4.29; mass < 10Msun). In the case of supernova 2004et we show that the yellow-supergiant progenitor candidate, originally identified in Canada France Hawaii Telescope images, is still visible ~3 years post-explosion in observations from the William Herschel Telescope. High-resolution Hubble Space Telescope and Gemini (North) adaptive optics late-time imagery reveal that this source is not a single yellow supergiant star, but rather is resolved into at least three distinct sources. We report the discovery of the unresolved progenitor as an excess of flux in pre-explosion Isaac Newton Telescope i'-band imaging. Accounting for the late-time contribution of the supernova using published optical spectra, we calculate the progenitor photometry as the difference between the pre- and post-explosion, ground-based observations. We find the progenitor was most likely a late K to late M-type supergiant of 8 +5/-1 Msun. In all cases we conclude that future, high-resolution observations of the supernova sites will be required to confirm these results.

Expanding Atmosphere of the M-Type Supergiant in VV Cephei

A search and radial-velocity measurements of forbidden lines were performed on spectrograms of VV Cep at 0.35–0.45 μm taken between 1976 and 1984, which was about half the orbital period around the last eclipse. We identified 13 [FeII] lines, 3 [NiII] lines, and one [CuII] line including at least six new lines on the spectra of VV Cep. The radial velocities of these forbidden lines were independent of the eclipse phase, and differed with respect to the transition probability and excitation potential. The forbidden-line features indicate the presence of gaseous motions expanding from the M-type supergiant with velocities of ∼ 20 km s−1. The mass-loss rate is estimated to be on the order of 10−6M⊙ yr−1.

Millimetre Observations of Evolved Stars

Radio emission at centimetre and millimetre wavelengths provides a powerful tool for studying the circumstellar envelopes of evolved stars. These include stars on the asymptotic giant branch (AGB), post-AGB stars and a small number of massive M-type supergiant stars. The AGB stars and M-type supergiants are characterised by extremely high mass-loss rates. The mass loss in such an evolved star is driven by radiation pressure acting on grains which form in the outer stellar atmosphere. The grains are accelerated outwards and transfer momentum to the gas through grain–gas collisions. The outflowing dust and gas thus form an expanding circumstellar envelope through which matter flows from the star to the interstellar medium, at a typical velocity of 15 km s−1. For a recent review of circumstellar mass loss see Chapman, Habing &amp; Killeen (1995).

UBV photoelectric photometry of the 1976–1978 eclipse of VV Cephei

This paper presents the result of UBV photoelectric photometry of VV Cephei. The contact times are determined from the B-V and U-B curves. The epoch of mideclipse is JD 2443361 in agreement with prediction. The unequal depths of the falling and rising branches of the colour curves can be explained by gas streaming from the M-type supergiant component. The radius of the M-type supergiant is about 1860 R and its atmosphere has a thickness of 450 R.

Red giants and supergiants with degenerate neutron cores

Results are presented for calculations of spherically symmetric stellar models with degenerate neutron cores which bear the same relationship to neutron stars as ordinary red giants have the white dwarfs. The external appearance of the models is shown to be similar to that of an ordinary M-type supergiant, while their evolutionary lifetime is ten times as long. The models are relativistic and nonrotating, containing a degenerate neutron core of 1 solar mass (radius of 10 km) surrounded by a nondegenerate massive diffuse envelope, with the core and envelope separated by a thin energy-producing layer. Effective temperatures and radii are calculated, and two families of models (red giants and supergiants) are described which are distinguished by their luminosities. Uncertainties and possible instabilities in the models are discussed. Possible formation mechanisms for such stars are hypothesized, including collapse of the degenerate electron core of a 'normal' giant or supergiant, supercritical accretion onto a neutron star, and spiralling of a neutron star in a close binary system into its companion's interior.

The Circumstellar Envelope of Alpha Herculis.

system of a Herculis, the M-type supergiant, and its visual companion, a giant G-type spectroscopic binary, are surrounded by a common circumstellar envelope. This envelope produces violet-displaced absorption cores in the zero-volt lines of the M star and stationary zero-volt absorption lines in the spectrum of the G star. The minimum radius of the envelope is 2 X 10 RO; its minimum mass is probably about 1 X 10- ®. It is condensed into clouds that fill only the fraction 10- of the whole spherical volume; within a condensation, the electron density is about 106 . The envelope is expanding at a velocity of 10 km/sec; the matter in it has been ejected by the M star and is being lost to the system at a probable rate of at least 3 X 10-8 ®/year. There is evidence for a comparable rate of mass loss from all other late-type supergiants. This process may be important in the evolution of all massive stars that have exhausted their hydrogen.

The M-Type Supergiant Members of the Double Cluster in Perseus.

The M-Type Supergiant Members of the Double Cluster in Perseus.