Pulsation Cycle Research Articles

VERA observations of SiO maser emission from R Aquarii

Aims. We aim to study the spatial distribution of the v = 1 and v = 2 J = 1-0 SiO maser emission from R Aqr, a well known stellar symbiotic system. In particular, we intend to determine the annual parallax and proper motion of the source by means of measurements of the absolute coordinates of the maser spots. Methods. We performed VLBI observations of the v = 1 and v = 2 J = 1-0 maser emission, at 7 mm wavelength, using VERA. We present observations made in 11 epochs, between December 2004 and October 2006, and observations by other authors are also discussed. VERA provides very high spatial resolution and accurate astrometric measurements, thanks to its double-beam observing system. From fitting ring-like structures to the maser spot distributions, we determine absolute J2000 coordinates of the central star. (SiO maser emission is known to be distributed around the star forming spot rings at a few stellar radii.) Results. Maps with accurate absolute coordinates were obtained in 8 epochs. From the coordinates determined for the central star, we measured parallax and linear proper motion. We obtain π = 4.7 ± 0.8 mas, compatible with, but much more accurate than, the Hipparcos value (5.07 ± 3.15 mas), and deduce a distance of R Aqr D = 214 +45 -32 pc. Our accurate astrometry also yields a reliable comparison between the spot distributions of both v = 1 and v = 2 J = 1-0 lines. We find that both masers come from similar regions, i.e. at similar distances from the star and defining common spot clusters, confirming previous results. But the coincidences between spots of both lines are very rare, within the spatial and spectral resolution of our experiments. This result is found for eight epochs, spanning more than one pulsation cycle (of the cool Mira-type component). We argue that explaining the finer details of the v = 1,2 J = 1-0 distributions found here would require new theoretical efforts. Finally, our observations also allow the study of the structure and dynamics of the close circumstellar shells in R Aqr. We do not confirm the previous suggestion that these shells are rotating; instead, we conclude that the observed kinematics is very probably caused by pulsations and random movements. The spatial distribution of the maser spots is found to be variable, but to show a stable axial symmetry.

First CoRoT light curves of RR Lyrae stars

Context. The CoRoT-Convection Rotation and planetary Transits-space mission is a great opportunity for monitoring stars with excellent time-sampling and unprecedented photometric precision for up to 150 days. As an important benefit, high-quality RR Lyrae light curves are obtained with a quasi-uninterrupted coverage over several pulsation and Blazhko cycles. Aims. The Blazhko effect in RR Lyrae stars is an unsolved problem of astrophysics. We used the high-precision space data to contribute more precise knowledge to explain the possible physical processes behind the phenomenon. Methods. We applied different period-finding techniques including Period04, MuFrAn, PDM and SigSpec. Amplitude and phase modulation were investigated by an analytical function method as well as with the traditional O-C diagrams. Results. The Blazhko modulation frequency is directly detected in the spectrum, as well as its first and second harmonics. It shows the non-linear nature of the Blazhko modulation. Besides the triplets, further higher-order modulation side peaks appear around the pulsation frequency as quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet structures. Additional frequencies, not belonging to the classical multiplet structures, are detected, as well as their linear combinations with the fundamental radial mode. We interpret these additional terms as non-radial modes. During the five consecutive Blazhko cycles, there is a shift of the maximum phase around 0.011 pulsation phase which is likely the consequence of a long term modulation.

THE PULSATION OF χ CYGNI IMAGED BY OPTICAL INTERFEROMETRY: A NOVEL TECHNIQUE TO DERIVE DISTANCE AND MASS OF MIRA STARS

We present infrared interferometric imaging of the S-type Mira star Chi Cygni. The object was observed at four different epochs in 2005-2006 with the IOTA optical interferometer (H band). Images show up to 40% variation in the stellar diameter, as well as significant changes in the limb darkening and stellar inhomogeneities. Model fitting gave precise time-dependent values of the stellar diameter, and reveals presence and displacement of a warm molecular layer. The star radius, corrected for limb darkening, has a mean value of 12.1 mas and shows a 5.1mas amplitude pulsation. Minimum diameter was observed at phase 0.94+/-0.01. Maximum temperature was observed several days later at phase 1.02+/-0.02. We also show that combining the angular acceleration of the molecular layer with CO (Delta v = 3) radial velocity measurements yields a 5.9+/-1.5 mas parallax. The constant acceleration of the CO molecules -- during 80% of the pulsation cycle -- lead us to argument for a free-falling layer. The acceleration is compatible with a gravitational field produced by a 2.1(+1.5/-0.7) solar mass star. This last value is in agreement with fundamental mode pulsator models. We foresee increased development of techniques consisting in combining radial velocity with interferometric angular measurements, ultimately allowing total mapping of the speed, density, and position of the diverse species in pulsation driven atmospheres.

Helium lines in RR Lyrae spectra

Context. During a comparative study of atmospheric phenomena that occur during the pulsation cycles of 10 RRab stars I unexpectedly found helium lines in emission and/or absorption in all 10 stars during rising light. The progression of events in the time-evolution of the helium spectrum differs in detail from star to star, but 9 of the 10 stars share a number of general characteristics, illustrated approximately by the behavior of RV Oct described in this Note. Aims. My long-term aim is to provide a comprehensive empirical description of atmospheric phenomena that take place during the pulsation cycles of RRab stars. The short-term goal of this paper is to inform readers that measurements of helium lines place new constraints on shock-wave models for RRab stars. Methods. Results presented here are based on measurements and analysis of observations made with the echelle spectrograph of the du Pont 2.5-m telescope at Las Campanas Observatory. Results. A general pattern of behavior emerges albeit with significant star-to-star variation. He I λ5376 (D3) appears first as a pure emission feature, unlike the Balmer lines for which the emission profile is always overlain initially by strong redward-displaced absorption. The phase of maximum D3 emission coincides approximately with that of Hα, but the duration of conspicuous D3 emission (in excess of the local continuum) is less than 15 mn, approximately half the duration of similarly defined Hα emission. The emission phase is followed by the appearance, first, of redward-displaced absorption, and shortly thereafter by a second violet-displaced absorption component that strengthens with advancing phase. Both absorption components gradually weaken and disappear after radial-velocity minimum, which lags luminosity maximum by no more than 0.01P as discussed in Sect. 3. However, bona fide He I λ5876 emission flux in excess of the local continuum flux reappears near maximum light for some stars in the sample and persists for about 45 mn. The radial velocities of the initial and final He I λ5876 emission features differ little from the time-average value of the metallic-line radial velocities for RV Octantis. However, the absorption features appear to distort the location of the emission centroid, when they are strong.

Observational constraints on the magnetic field of RR Lyrae stars

Context. A high percentage of the astrophysically important RR Lyrae stars show a periodic amplitude and/or phase modulation of their pulsation cycles. More than a century after its discovery, this “Blazhko effect” still lacks acceptable theoretical understanding. In one of the plausible models for explaining the phenomenon, the modulation is caused by the effects of a magnetic field. So far, the available observational data have not allowed us to either support nor rule out the presence of a magnetic field in RR Lyrae stars. Aims. We intend to determine whether RR Lyrae stars are generally characterized by the presence of a magnetic field organized on a large scale. Methods. With the help of the FORS1 instrument at the ESO VLT we performed a spectropolarimetric survey of 17 relatively bright southern RR Lyrae stars, both Blazhko stars and non-modulated stars, and determined their mean longitudinal magnetic field with a typical error bar Results. All our measurements of the mean longitudinal magnetic field resulted in null detections within 3 σ . From our data we can set an upper limit for the strength of the dipole component of the magnetic fields of RR Lyrae stars to ~130 G. Because of the limitations intrinsic to the diagnostic technique, we cannot exclude the presence of higher order multipolar components. Conclusions. The outcome of this survey clarifies that the Blazhko modulation in the pulsation of RR Lyrae stars is not correlated with the presence of a strong, quasi-dipolar magnetic field.

Milk flow-controlled changes of pulsation ratio and pulsation rate affect milking characteristics in dairy cows

To test a system with milk flow-controlled pulsation, milk flow was recorded in 29 Holstein cows during machine milking. The three different treatments were routine milking (including a pre-stimulation of 50-70 s), milking with a minimum of teat preparation and milking with milk flow-controlled b-phase, i.e. with a gradually elongated b-phase of the pulsation cycle with increasing milk flow rate and shortening again during decreasing milk flow. For data evaluation the herd was divided into three groups based on the peak flow rate at routine milking (group 1: <3.2 kg/min; group 2: 3.2-4.5 kg/min; group 3: >4.5 kg/min). Compared with routine milking, milking with milk flow-controlled b-phase caused a significant elevation of the peak flow rate and the duration of incline lasted longer especially in cows with a peak flow rate of >3.2 kg/min in routine milking. In milking with a minimum of teat preparation the duration of incline lasted longer compared with the two other treatments. Bimodality of milk flow, i.e. delayed milk ejection at the start of milking, was most frequent at milking with a minimum of teat preparation. No significant differences between routine milking and milking with milk flow-controlled b-phase were detected for all other milking characteristics. In summary, milking with milk flow-controlled b-phase changes the course of milk removal, however mainly in cows with high peak flow rates.

Wave processes in column rods of a rocker machine

The propagation of stress waves in a column of rods of a rocker machine during a lift is considered at an arbitrary time instant. Using an integral transformation of time, the second-degree differential equation for displacement is solved where the force of the column dead weight is taken into account. It is shown that the stress variation is of a pulsating type; parameters and a form of the pulsation cycle are given. The fatigue strength of a thread in the union joint of rods and the lifetime of rods having an initial fracture are determined.

20319 血液ポンプ羽根車のための翼特性および翼周り流れの基礎的研究(人工臓器と医用工学(4),OS.8 人工臓器と医用工学)

A rotary blood pump designed by traditional wing theory could get remarkably low performance in pulsatile flow as compared to that in steady (nonpulsatile) flow. This is because the wing theory is not suitable for low Reynolds number and pulsatile flow like blood flow in which the pump is actually used. In the present study, investigation of a low Reynolds number and pulsatile isolated wing flow is carried out by computational fluid dynamics (CFD) and construction of experimental equipment as fundamental study. The CFD results indicated that the large scale separation on suction surface of the wing occurred during the almost all one cycle of pulsation by comparing the pulsatile and nonpulsatile inflow conditions. In order to verify the CFD results and to clarify hydraulic characteristics of low Reynolds number of 10^3 or 10^4 and pulsatile wing flow, it is constructed water tunnel testing system by which it can measure fluid forces acting on the wing in the condition of low speed flow and both pulsatile and nonpulsatile flow.

Time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa)

Aims. We present observations and analysis of time-series spectroscopy and photometry of the pulsating subdwarf B star PG 1219+534 (KY UMa). Subdwarf B stars are blue horizontal branch stars that have shed most of their hydrogen envelopes. Pulsating subdwarf B stars allow a probe into this interesting phase of evolution. Methods. Low-resolution spectra were obtained at the Nordic Optical Telescope and Kitt Peak National Observatory, and photometric observations were obtained at MDM and Baker observatories in 2006. We extracted radial velocity and equivalent width variations from several Balmer and He I lines in individual spectra. The pulsation frequencies were separated via phase binning to detect lineprofile variations in Balmer and helium lines, which were subsequently matched to atmospheric models to infer effective temperature and gravity changes throughout the pulsation cycle. Results. From the photometry we recovered the four previously observed frequencies and detected a new fifth frequency. From the spectra we directly measured radial velocity and equivalent width variations for the four main frequencies and from atmospheric models. We successfully inferred temperature and gravity changes for these four frequencies. We compared amplitude ratios and phase differences of these quantities and searched for outliers that could be identified as high-degree modes. These are the first such measurements for a “normal” amplitude pulsating subdwarf B star, indicating that spectroscopic studies can benefit the majority of pulsating subdwarf B stars.

Time-resolved high-resolution spectroscopy of the bright pulsating subdwarf B star Balloon 090100001

We have obtained, for the first time, a time-resolved high-resolution spectroscopic data set of the bright sdBV Balloon 090100001. In the autumn of 2006 we gathered over 1600 time-resolved high-resolution spectra with the aim to identify the pulsation modes in Balloon 090100001 by studying the line-profile variations. In this paper we present the results obtained from 842 spectra secured in August 2006, aiming to identify the dominant pulsation mode in this star. We phase fold the spectra onto the known dominant period of 2.80746 mHz, to study the abundances and line-broadening as a function of pulsation phase. We find that the abundances do not stand out against those of other subdwarf B stars. Consequently, there is no way to demonstrate that the richness of the frequency spectrum and/or the high pulsation amplitudes are related to abundance effects. The abundances do not change with pulsational phase. The metal lines in Balloon 090100001 are much broader than for non-pulsating subdwarfs, and during the cycle of the main pulsation the line broadening varies with ∼2.5 km s −1 amplitude. Hence, we find clear evidence of pulsational broadening of the lines. We combine the information content of 56 narrow absorption lines through a cross-correlation technique, to create cross-correlation functions that for each individual pulsation phase approximate the average line-profile shape. The resulting profiles have a sufficient signal-to-noise ratio for a mode-identification analysis. From the cross-correlation analysis we find that the pulsation amplitude of the main mode decreased from 19 km s −1 in 2004 to 14.5 km s −1 in 2006. We present, for an sdB star, the first application of pulsation-mode identification based on line-profile variations of lines of heavy elements. To fit the line profiles we consider all modes with degree � ≤ 2 and associated azimuthal order m, and we use a model of the pulsational surface-velocity field. The models with � = 2 do not fit the profiles well, and consequently we can rule out quadrupole modes as the origin of the main pulsation mode in Balloon 090100001. Best fits are obtained from the model of a radial pulsation (� = 0) and the model of a dipole pulsation (� = 1).

Kinematic properties of the jellyfish Aurelia sp.

A new, relatively simple method for determining the kinematic properties of jellyfish is presented. The bell movement of the scyphomedusa (Aurelia sp.) during its pulsation cycle was analysed using computer-aided visualization. Sequences of video images of individual Aurelia in a large aquarium were taken using a standard video camera. The images were then processed to obtain time series of the relative positions of selected points on the surface of the medusa’s bell. The duration of the bell relaxation was longer than that of the bell contraction, thereby confirming published results. In addition, the area of the exumbrellar surface of Aurelia increased during bell relaxation by more than 1.3-times that of the exumbrellar surface area during the maximum contraction of the bell. The volume change during the bell pulsation cycle was also measured using the same visualization method. Significant changes, of up to 50%, in the subumbrellar cavity volume were revealed while, in contrast, the volume between the exumbrellar and subumbrellar surfaces generally remained unchanged during the entire pulsation cycle of the bell. Comparison of the time series of the exumbrellar surface area and of the subumbrellar cavity volume indicated that the change of volume takes place before the change of the surface area of the bell.

Numerical Study of Heat Transfer of a Porous-Block-Mounted Heat Source Subjected to Pulsating Channel Flow

A numerical analysis was conducted to investigate the convective characteristics of pulsating flow through a channel with a porous-block-mounted heat source. Comprehensive time-dependent flow and temperature data are calculated and averaged over a pulsation cycle in a periodic steady state. The impacts of the Darcy number, pulsating frequency and amplitude, and porous blockage ratio are documented in detail. The results indicate that the periodic alteration in the structure of recirculation flows, caused by both porous block and flow pulsation, has a direct impact on the flow behavior in the vicinity of the porous block and on the heat transfer rate from the heater.

The effects of pulse frequency and duty cycle on the skin friction downstream of pulsed jet vortex generators in an adverse pressure gradient turbulent boundary layer

An investigation on the viability of pulsed jets as active vortex generator devices was conducted. The devices were installed and tested on an adverse pressure gradient turbulent boundary layer designed to simulate the suction side of a conventional aircraft wing. Both co-rotating and counter-rotating jet geometries were used. The duty cycle and frequency of pulsation were varied and their effects were investigated by measuring the skin friction gains at a predefined location (the location of the minimum skin friction for the un-actuated situation) on the adverse pressure gradient turbulent boundary layer. Pulsing the jets proved to be successful in increasing the wall skin friction and therefore potentially delaying separation. The improvements in wall shear stress were approximately proportional to the duty cycle. The frequency of jet pulsation was found to be important for attaining optimal gains, however no clear relationship between frequency and shear stress gain was observed. Phase averaged wall shear stress measurements far downstream of actuation indicate that quasi-steady structures are introduced by the vortex generators when actuating with a sufficiently high pulse frequency. In this situation interactions between successive structures produced by the jets were likely to be occurring.

The Keck Aperture Masking Experiment: Multiwavelength Observations of Six Mira Variables

The angular diameters of six oxygen-rich Mira-type long-period variables have been measured at various NIR wavelengths using the aperture-masking technique in an extensive observing program from 1997 January to 2004 September. These data sets span many pulsation cycles of the observed objects and represent the largest study of multiwavelength, multiepoch interferometric angular diameter measurements on Mira stars to date. The calibrated visibility data of ο Cet, R Leo, R Cas, W Hya, χ Cyg, and R Hya are fitted using a uniform disk brightness distribution model to facilitate comparison between epochs, wavelengths, and with existing data and theoretical models. The variation of angular diameter as a function of wavelength and time is studied, and cyclic diameter variations are detected for all objects in our sample. These variations are believed to stem from time-dependent changes of density and temperature (and hence varying molecular opacities) in different layers of these stars. The similarities and differences in behavior between these objects are analyzed and discussed in the context of existing theoretical models. Furthermore, we present time-dependent 3.08 μm angular diameter measurements, probing for the first time these zones of probable dust formation, which show unforeseen sizes and are consistently out of phase with other NIR layers shown in this study. The S-type Mira χ Cyg exhibits significantly different behavior compared to the M-type Mira variables in this study, in both its NIR light curves and its diameter pulsation signature. Our data show that the NIR diameters predicted by current models are too small and need to incorporate additional and/or enhanced opacity mechanisms. Also, new tailored models are needed to explain the behavior of the S-type Mira χ Cyg.

A Compact Dust Shell in the Symbiotic System HM Sagittae

We present high spatial resolution observations of the mid-infrared core of the dusty symbiotic system HM Sge. The MIDI interferometer was used with the VLT Unit Telescopes and Auxiliary Telescopes providing baselines oriented from PA=42° to 105°. The MIDI visibilities are compared with the ones predicted in the frame of various spherical dust shells published in the literature involving single or double dusty shells intended to account for the influence of the hot White Dwarf. The mid-IR environment is unresolved by a 8m telescope and the MIDI spectrum exhibits a level similar to the ISO spectra recorded 10 yr ago. The discrepancies between the HWHM at different angle orientations suggest an increasing level of asymmetry from 13 to 8 μ m. The observations are surprisingly well fitted by the densest (optically thick in the N band) and smallest spherical model published in the literature based on the ISO data, although such a model does not account for the variations of near-IR photometry due to the Mira pulsation cycle suggesting a much smaller optical thickness. These observations also discard the two shells models, developed in an attempt to take into account the effect of the White Dwarf illumination onto the dusty wind of the Mira. These models are too extended, and lead to a level of asymmetry of the dusty environment tightly constrained by the MIDI visibilities. These observations show that a high rate of dust formation is occurring in the vicinity of the Mira which seems to be not highly perturbed by the hot companion.

Fiber-based multispeckle detection for time-resolved diffusing-wave spectroscopy: characterization and application to blood flow detection in deep tissue

We present a technique for the measurement of temporal field autocorrelation functions of multiply scattered light with subsecond acquisition time. The setup is based on the parallel detection and autocorrelation of intensity fluctuations from statistically equivalent but independent speckles using a fiber bundle, an array of avalanche photodiodes, and a multichannel autocorrelator with variable integration times between 6.5 and 104 ms. Averaging the autocorrelation functions from the different speckles reduces the integration time in diffusing-wave spectroscopy experiments drastically, thus allowing us to resolve nonstationary scatterer dynamics with single-trial measurements. We present applications of the technique to the measurement of arterial and venous blood flow in deep tissue. We find strong deviations both of the shape and characteristic decay time of autocorrelation functions recorded at different phases of the pulsation cycle from time-averaged autocorrelation functions.

Spectroscopy of the hot pulsating star β Cephei. Velocities and EWs from C, N, O and Si lines

Frequencies in oscillating β Cephei stars are usually inferred by means of radial velocities measured from the Si iii triplet λλ 4552–4574 Å. These lines, relatively insensitive to the variation of T eff through a pulsation cycle, show small equivalent width variations. In this study, we aimed to verify if the behavior of radial velocities and equivalent widths measured from other ions are compatible with the one observed from Si iii lines and than to verify the possible vertical stratification along the stellar atmosphere. For this reason we selected from our spectra a number of, unblended and well isolated, C ii, N ii and O ii lines besides the famous Si iii triplet. All those lines cover the range in optical depth between −2.1 and −0.5. Unfortunately, we did not find any differences in the radial velocities behavior line-by-line and then we derived the frequency of the principal radial mode combining all the velocities derived from each spectral line separately. The inferred frequency was f 1 = 5.249677 ± 0.000007 c d −1. Another important task we would like to accomplish with this paper is to make available to the community our large sample of spectroscopic data, that is 932 velocities and equivalent widths measured from our sample of C, N, O and Si lines. All the spectra were acquired at the 1-m class telescope of the stellar station of the INAF – Osservatorio Astrofisico di Catania, in the period starting from July, the 27th 2005 to November, the 1st 2006.

A high-precision search for magnetic field oscillations in the roAp star HD 24712

We have obtained a time series of 81 high-cadence circular polarization observations of the rapidly oscillating Ap star HD 24712 with the new ESPaDOnS spectropolarimeter at CFHT. We used the high-S/N, high-resolution Stokes I and V spectra to investigate possible variation of the mean longitudinal field over the pulsation cycle in this roAp star. Our multiline magnetic field and radial velocity measurements utilized 143 spectral lines of rare-earth elements, attaining precision better than 13 G and 19 m s -1 , respectively. A multiperiodic radial velocity variation with an amplitude of 40–136 m s -1 is clearly detected at the known pulsation frequencies of HD 24712. At the same time, no evidence for pulsational changes of the magnetic field can be found. We derive a $3\sigma$ upper limit of 10 G, or about 1% of the mean longitudinal field strength, for magnetic field oscillations in the upper atmosphere of HD 24712. The absence of detectable pulsational variability of the magnetic field provides a valuable constraint for the interaction between pulsations and magnetic field in roAp stars and is compatible with the recent predictions of detailed theoretical models of stellar magnetoacoustic oscillations.

Near infrared spectroscopy of the HADS stars V703 Sco and 1 Mon: First steps towards the use of near-IR spectroscopy for the study of δ Scuti stars

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind veränderliche und pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. Die Arbeiten in Vol.151 berichten über ein breites Spektrum der asteroseismologischen Forschung: Neben Beiträgen über die erdgebundene photometrische sowie spektroskopische Beobachtung von pulsierenden Sternen werden auch die neuesten Ergebnisse aus den Beobachtungen mit dem MOST-Satelliten besprochen. Theoretische Arbeiten über Konvektion und Opazitäten ergänzen diese Ausgabe. Contents: D. B. Guenther, T. Kallinger, P. Reegen, W. W. Weiss, J. M. Matthews, R. Kuschnig, A. F. J. Moffat, S. M. Rucinski, D. Sasselov and G. A. H. Walker: Searching for ρ-modes in η Boötis & Procyon using MOST satellite data; L. Fox Machado; J. H. Peña, G. Muñoz and B. Vargas: On the nature of the δ Scuti star star HD 115520; D. Sinachopoulos and P. Gavras: Photometric observations and frequency analysis of the δ Scuti NT Hya; J. C. Suárez, E. Michel, G. Houdek, F. Pérez Hernández and Y. Lebreton: Asteroseismology of δ Scuti Stars in open clusters: Praesepe; J. C. Suárez, R. Garrido and A. Moya: Analysis of II1/0 (Ω) period ratios in the presence of near degeneracy; A. Miglio, J. Montalbán and M.-A. Dupret: Revised instability domains of SPB and β Cephei stars; P. J. Amado: Near infrared spectroscopy of the HADS stars V703 Sco and 1 Mon: First steps towards the use of near-IR spectroscopy for the study of δ Scuti stars; C. Schoenaers and A. E. Lynas-Gray: A new slowly pulsating subdwarf-B star: HD 4539; M. Breger and P. Beck: Call for papers for the next issue of CoAst

On the CO Near‐Infrared Band and the Line‐splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae

We report on multiepoch optical and near-infrared spectroscopy around the first-overtone rovibrational band of CO in the pulsating yellow hypergiant ρ Cas, one of the most massive stars in the Galaxy and a candidate SN II progenitor. We argue that the double cores of the CO absorption lines, which have previously been attributed to separate circumstellar shells expelled during its recurrent outbursts, result in fact from a superposition of a wide absorption line and a narrow central emission line. The CO line doubling returns over subsequent pulsation cycles, where the superposed line emission assumes its largest intensity near phases of maximum light. We find that the morphology and behavior of the CO band closely resemble the remarkable line-splitting phenomenon also observed in optical low-excitation atomic lines. Based on radiative transport calculations, we present a simplified model of the near-infrared CO emission emerging from cooler atmospheric layers in the immediate vicinity of the photosphere. We speculate that the kinetic temperature minimum in our model results from a periodic pulsation-driven shock wave. We further discuss a number of alternative explanations for the origin of the ubiquitous emission-line spectrum, possibly due to a quasi-chromosphere or a steady shock wave at the interface of a fast expanding wind and the interstellar medium. We present a number of interesting spectroscopic similarities between ρ Cas and other types of cool variable supergiants, such as the RV Tau and R CrB stars. We further propose a possibly common mechanism for the enigmatic outburst behavior of these luminous pulsating cool stars.