Abstract Digest #3

Due to the deadline for proceeding contributions to IAUS 307, held in Geneva, and the large number of preprints from that conference, those contribtuions will be published in an extra edition in a few days. Today's abstracts:

  • "The environment of the fast rotating star Achernar. III. Photospheric parameters revealed by the VLTI" by A. Domiciano de Souza et al.
  • "Beyond the diffraction limit of optical/IR interferometers. II. Stellar parameters of rotating stars from differential phases" by M. Hadjara et al.
  • "Study of the sub-AU disk of the Herbig B[e] star HD 85567 with near-infrared interferometry" by J. Vural at al.
  • "HD314884: A Slowly Pulsating B star in a Close Binary" by Christopher B. Johnson et al.
  • "Time-dependent modeling of extended thin decretion disks of critically rotating stars" by Petr Kurfürst, Achim Feldmeier, Jirí Krticka
  • "New objects with the B[e] phenomenon in the Large Magellanic Cloud" by H. Levato, A.S. Miroshnichenko, C. Saffe
  • "Asteroseismology revealing trapped modes in KIC 10553698A" by R. H. Ostensen et al.
  • "Finding eta Car Analogs in Nearby Galaxies Using Spitzer: II. Identification of An Emerging Class of Extragalactic Self-Obscured Stars" by Rubab Khan et al.
  • "eta Carinae Baby Homunculus Uncovered by ALMA" by Zulema Abraham, Diego Falceta-Gonçalves, Pedro P. B. Beaklin

The environment of the fast rotating star Achernar. III. Photospheric parameters revealed by the VLTI

A. Domiciano de Souza et al.

Context. Rotation significantly impacts on the structure and life of stars. In phases of high rotation velocity (close to critical), the photospheric structure can be highly modified, and present in particular geometrical deformation (rotation flattening) and latitudinal- dependent flux (gravity darkening). The fastest known rotators among the nondegenerate stars close to the main sequence, Be stars, are key targets for studying the effects of fast rotation on stellar photospheres. Aims. We seek to determine the purely photospheric parameters of Achernar based on observations recorded during an emission-free phase (normal B phase). Methods. Several recent works proved that optical/IR long-baseline interferometry is the only technique able to sufficiently spatially resolve and measure photospheric parameters of fast rotating stars. We thus analyzed ESO-VLTI (PIONIER and AMBER) interferometric observations of Achernar to measure its photospheric parameters by fitting our physical model CHARRON using a Markov chain Monte Carlo method. This analysis was also complemented by spectroscopic, polarimetric, and photometric observations to investigate the status of the circumstellar environment of Achernar during the VLTI observations and to cross-check our model-fitting results. Results. Based on VLTI observations that partially resolve Achernar, we simultaneously measured five photospheric parameters of a Be star for the first time: equatorial radius (equatorial angular diameter), equatorial rotation velocity, polar inclination, position angle of the rotation axis projected on the sky, and the gravity darkening beta coefficient (effective temperature distribution). The close circumstellar environment of Achernar was also investigated based on contemporaneous polarimetry, spectroscopy, and interferometry, including image reconstruction. This analysis did not reveal any important circumstellar contribution, so that Achernar was essentially in a normal B phase at least from mid-2009 to end-2012, and the model parameters derived in this work provide a fair description of its photosphere. Finally, because Achernar is the flattest interferometrically resolved fast rotator to-date, the measured beta and flattening, combined with values from previous works, provide a crucial test for a recently proposed gravity darkening model. This model offers a promising explanation to the fact that the measured beta parameter decreases with flattening and shows significantly lower values than the classical prediction of von Zeipel.

Available at: A&A in press


Beyond the diffraction limit of optical/IR interferometers. II. Stellar parameters of rotating stars from differential phases

M. Hadjara et al.

Context. As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by spectrally resolved long baseline interferometry using earth-rotation synthesis. Aims. We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters. Methods. We used differential phase data from the AMBER/VLTI instrument obtained prior to refurbishing its spectrometer in 2010. With the exception of Fomalhaut, which has been observed in the medium-resolution mode of AMBER (R approx 1500), our three other targets, Achernar, Altair, and delta Aquilae offered high-resolution (R approx 12000) spectro-interferometric data around the Br-gamma absorption line in K band. These data were used to constrain the input parameters of an analytical, still realistic model to interpret the observations with a systematic approach for the error budget analysis in order to robustly conclude on the physics of our 4 targets. We applied the super resolution provided by differential phases phi_diff to measure the size (equatorial radius Req and angular diameter / eq ), the equatorial rotation velocity (Veq), the inclination angle (i), and the rotation axis position angle (PArot) of 4 fast-rotating stars: Achernar, Altair, delta Aquilae, and Fomalhaut. The stellar parameters of the targets were constrained using a semi-analytical algorithm dedicated to fast rotators SCIROCCO. Results. The derived parameters for each star were Req = 11.2 ±0.5Rsun, Veq sin i = 290 ± 17km.s-1, PArot = 35.4deg ± 1.4deg, for Achernar; Req = 2.0 ± 0.2Rsun, Veq sin i = 226 ± 34km.s-1, PArot = -65.5deg ± 5.5deg, for Altair; Req = 2.2 ± 0.3Rsun, Veq sin i = 74 ± 35km.s-1, PArot = -101.2deg ± 14deg, for delta Aquilae; and Req = 1.8 ± 0.2Rsun, Veq sin i = 93 ± 16km.s-1, PArot = 65.6deg ± 5deg , for Fomalhaut. They were found to be compatible with previously published values from differential phase and visibility measurements, while we were able to determine, for the first time, the inclination angle i of Fomalhaut (i = 90deg ± 9deg) and delta Aquilae (i = 81deg ± 13deg), and the rotation-axis position angle PArot of delta Aquilae. Conclusions. Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution.

Available at: A&A in press


Study of the sub-AU disk of the Herbig B[e] star HD 85567 with near-infrared interferometry

J. Vural at al.

Context. The structure of the inner disk of Herbig Be stars is not well understood. The continuum disks of several Herbig Be stars have inner radii that are smaller than predicted by models of irradiated disks with optically thin holes. Aims. We study the size of the inner disk of the Herbig B[e] star HD 85567 and compare the model radii with the radius suggested by the size-luminosity relation. Methods. The object was observed with the AMBER instrument of the Very Large Telescope Interferometer. We obtained K-band visibilities and closure phases. These measurements are interpreted with geometric models and temperature-gradient models. Results. Using several types of geometric star-disk and star-disk-halo models, we derived inner ring-fit radii in the K band that are in the range of 0.8–1.6 AU. Additional temperature-gradient modeling resulted in an extended disk with an inner radius of 0.67(+0.51±0.21) AU, a high inner temperature of 2200(+750±350) K, and a disk inclination of 53(+15-11)deg. Conclusions. The derived geometric ring-fit radii are approximately 3–5 times smaller than that predicted by the size-luminosity relation. The small geometric and temperature-gradient radii suggest optically thick gaseous material that absorbs stellar radiation inside the dust disk.

Available at: A&A in press


HD314884: A Slowly Pulsating B star in a Close Binary

Christopher B. Johnson et al.

We present the results of a spectroscopic and photometric analysis of HD314884, a slowly pulsating B star (SPB) in a binary system with detected soft X-ray emission. We spectrally classify the B star as a B5V-B6V star with T$_{eff}$ = 15,490 $\pm$ 310 K, log $g$ = 3.75 $\pm$ 0.25 dex, and a photometric period of P$_{0}$ = 0.889521(12) days. A spectroscopic period search reveals an orbital period for the system of P$_{orb}$ = 1.3654(11) days. The discrepancy in the two periods and the identification of a second and third distinct frequency in the photometric fourier transform at P$_1$ = 3.1347(56) and P$_2$ = 1.517(28) days provides evidence that HD314884 is a slowly pulsating B star (SPB) with at least three oscillation frequencies. These frequencies appear to originate from higher-order, non-linear tidal pulsations. Using the dynamical parameters obtained from the radial velocity curve, we find the most probable companion mass to be M$_1$ = $\sim$0.8 M$_{\odot}$ assuming a typical mass for the B star and most probable inclination. We conclude that the X-ray source companion to HD314884 is most likely a coronally active G-type star or a white dwarf (WD), with no apparent emission lines in the optical spectrum. The mass probability distribution of the companion star mass spans 0.6-2.3 M$_{\odot}$ at 99$\%$ confidence which allows the possibility of a neutron star companion. The X-ray source is unlikely to be a black hole unless it is of a very low mass or low binary inclination.

Available at: arXiv:1407.7938


Time-dependent modeling of extended thin decretion disks of critically rotating stars

Petr Kurfürst, Achim Feldmeier, Jirí Krticka

During their evolution massive stars can reach the phase of critical rotation when a further increase in rotational speed is no longer possible. Direct centrifugal ejection from a critically or near-critically rotating surface forms a gaseous equatorial decretion disk. Anomalous viscosity provides the efficient mechanism for transporting the angular momentum outwards. The outer part of the disk can extend up to a very large distance from the parent star. We study the evolution of density, radial and azimuthal velocity, and angular momentum loss rate of equatorial decretion disks out to very distant regions. We investigate how the physical characteristics of the disk depend on the distribution of temperature and viscosity. We calculated stationary models using the Newton-Raphson method. For time-dependent hydrodynamic modeling we developed the numerical code based on an explicit finite difference scheme on an Eulerian grid including full Navier-Stokes shear viscosity. The sonic point distance and the maximum angular momentum loss rate strongly depend on the temperature profile and are almost independent of viscosity. The rotational velocity at large radii rapidly drops accordingly to temperature and viscosity distribution. The total amount of disk mass and the disk angular momentum increase with decreasing temperature and viscosity. The time-dependent one-dimensional models basically confirm the results obtained in the stationary models as well as the assumptions of the analytical approximations. Including full Navier-Stokes viscosity we systematically avoid the rotational velocity sign change at large radii. The unphysical drop of the rotational velocity and angular momentum loss at large radii (present in some models) can be avoided in the models with decreasing temperature and viscosity.

Available at: arXiv:1407.7952


New objects with the B[e] phenomenon in the Large Magellanic Cloud

H. Levato, A.S. Miroshnichenko, C. Saffe

Aims. The study is aimed at discovering new objects with the B[e] phenomenon in the Large Magellanic Cloud. Methods. We report medium-resolution optical spectroscopic observations of two newly found (ARDB 54 and NOMAD 0181-0125572) and two previously known (Hen S–59 and Hen S–137) supergiants with the B[e] phenomenon in the Large Magellanic Cloud. The observations were obtained with the GMOS spectrograph at the southern Gemini telescope. Results. The optical spectra and fundamental parameters of ARDB 54 and NOMAD 0181-0125572 are presented for the first time. We found that the Balmer line profiles of Hen S–59 and Hen S–137 were different from those observed in their spectra nearly 20 years ago. We suggest a higher effective temperature and luminosity for both objects. With the new fundamental parameters, the lowest luminosity for known supergiants with the B[e] phenomenon in the Magellanic Clouds is higher that previously thought (log L/Lsun ~ 4.5 instead of 4.0). The object Hen S–59 may be a binary system based on its UV excess, variable B - V color-index and radial velocity of emission lines, and periodically variable I–band brightness.

Available at: A&A in press


Asteroseismology revealing trapped modes in KIC 10553698A

R. H. Ostensen et al.

The subdwarf-B pulsator, KIC 10553698A, is one of 16 such objects observed with one-minute sampling for most of the duration of the Kepler Mission. Like most of these stars, it displays a rich g-mode pulsation spectrum with several clear multiplets that maintain regular frequency splitting. We identify these pulsation modes as components of rotationally split multiplets in a star rotating with a period of ~41 d. From 162 clearly significant periodicities, we are able to identify 156 as likely components of l = 1 or l = 2 multiplets. For the first time we are able to detect l = 1 modes that interpose in the asymptotic period sequences and that provide a clear indication of mode trapping in a stratified envelope, as predicted by theoretical models. A clear signal is also present in the Kepler photometry at 3.387 d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8 km/s. We find that the radial-velocity variations and the photometric signal have phase and amplitude that are perfectly consistent with a Doppler-beaming effect and conclude that the unseen companion, KIC 10553698B, must be a white dwarf most likely with a mass close to 0.6 Msun.

Available at: A&A in press


Finding eta Car Analogs in Nearby Galaxies Using Spitzer: II. Identification of An Emerging Class of Extragalactic Self-Obscured Stars

Rubab Khan et al.

Understanding the late-stage evolution of the most massive stars such as $\eta$ Carinae is challenging because no true analogs of $\eta$ Car have been clearly identified in the Milky Way or other galaxies. In Khan et. al. (2013), we utilized Spitzer IRAC images of $7$ nearby ($\lesssim4$ Mpc) galaxies to search for such analogs, and found $34$ candidates with flat or red mid-IR spectral energy distributions. Here, in Paper II, we present our characterization of these candidates using multi-wavelength data from the optical through the far-IR. Our search detected no true analogs of $\eta$ Car, which implies an eruption rate that is a fraction $0.01\lesssim F \lesssim 0.19$ of the ccSN rate. This is roughly consistent with each $M_{ZAMS} \gtrsim 70M_\odot$ star undergoing $1$ or $2$ outbursts in its lifetime. However, we do identify a significant population of $18$ lower luminosity $\left(\log(L/L_\odot)\simeq5.5-6.0\right)$ dusty stars. Stars enter this phase at a rate that is fraction $0.09 \lesssim F \lesssim 0.55$ of the ccSN rate, and this is consistent with all $25 < M_{ZAMS} < 60M_\odot$ stars undergoing an obscured phase at most lasting a few thousand years once or twice. These phases constitute a negligible fraction of post-main sequence lifetimes of massive stars, which implies that these events are likely to be associated with special periods in the evolution of the stars. The mass of the obscuring material is of order $\sim M_\odot$, and we simply do not find enough heavily obscured stars for theses phases to represent more than a modest fraction ($\sim 10\%$ not $\sim 50\%$) of the total mass lost by these stars. In the long term, the sources that we identified will be prime candidates for detailed physical analysis with JWST.

Available at: arXiv:1407.7530


eta Carinae Baby Homunculus Uncovered by ALMA

Zulema Abraham, Diego Falceta-Gonçalves, Pedro P. B. Beaklin

We report observations of eta Carinae obtained with ALMA in the continuum of 100, 230, 280, and 660 GHz in 2012 November, with a resolution that varied from 2.''88 to 0.''45 for the lower and higher frequencies, respectively. The source is not resolved, even at the highest frequency; its spectrum is characteristic of thermal bremsstrahlung of a compact source, but different from the spectrum of optically thin wind. The recombination lines H42alpha, He42alpha, H40alpha, He40alpha, H50beta, H28alpha, He28alpha, H21alpha, and He21alpha were also detected, and their intensities reveal non-local thermodynamic equilibrium effects. We found that the line profiles could only be fit by an expanding shell of dense and ionized gas, which produces a slow shock in the surroundings of eta Carinae. Combined with fittings to the continuum, we were able to constrain the shell size, radius, density, temperature, and velocity. The detection of the He recombination lines is compatible with the high-temperature gas and requires a high-energy ionizing photon flux, which must be provided by the companion star. The mass-loss rate and wind velocity, necessary to explain the formation of the shell, are compatible with an luminous blue variable eruption. The position, velocity, and physical parameters of the shell coincide with those of the Weigelt blobs. The dynamics found for the expanding shell correspond to matter ejected by eta Carinae in 1941 in an event similar to that which formed the Little Homunculus; for that reason, we called the new ejecta the "Baby Homunculus."

Available at: ApJ 791, 95


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