Searcharxiv⌕ Search

arXiv subjects

M. Cure

Publications and source records attributed to M. Cure.

17 recordsLinked to original sources
arXiv2026

New constraints on the binary system HD 327083 and its gaseous and dusty environments

Binary systems with circumbinary molecular and dust rings are of great interest because they provide insights into the dynamics and evolution of stellar systems and the chemistry of the surrounding material. We aim to elucidate the nature of the B[e] binary system HD 327083 and the physical properties of its circumstellar and circumbinary envelopes. We combined multi-epoch high-spectral resolution optical and near-IR observations with VLTI/MIDI observations and ASAS-3 photometry. The data were analysed to obtain the orbital parameters, study individual members of the system, and derive the properties of the atomic and molecular gas and the dusty components of the complex environment of HD 327083. We improved the orbital solution of HD 327083 and derived a period of P = 107.699 d. Most optical lines display variations in shape and intensity when folded with the orbital motion. We assigned F6 II-III and B1 spectral types respectively to the cool and hot components. A ring of molecular gas revolves around the system and varies with the orbital phase. The MIDI data show a silicate band in absorption at 9.7 micron and indicate that the dust distribution has an elliptical shape, with its inner edge varying with orbital phase between 12.5 and 44 AU. We conclude that the B[e] binary system HD 327083 comprises a B-type massive accreting star, and an F-type companion filling over 60% of its Roche lobe and evolving towards the red supergiant stage. The deformed shape of the F-type star causes a broad minimum in the light curve. The hot companion (26 000 K, log g = 3.0) is hidden by a compact disc and shows a bi-polar outflow. The entire binary system presents an O-rich environment. It is surrounded by warm CO and SiO molecular rings, which are enclosed by an elliptical dust structure. All these components were formed by the evolved donor star during its evolution towards the red supergiant phase.

astro-ph.SR↗
arXiv2016

Clumpy molecular structures revolving the B[e] supergiant MWC 137

The peculiar emission-line star MWC 137 with its extended optical nebula was recently classified as B[e] supergiant. To study the spatial distribution of its circumstellar molecular gas on small and large scales, we obtained near-infrared and radio observations using SINFONI and APEX, respectively. We find that the hot CO gas is arranged in moving clumpy ring and shell structures close to the star, while a cold CO envelope is encircling the borders of the optical nebula from the south to the west.

astro-ph.SR↗
arXiv2016

A method to deconvolve stellar rotational velocities II

Knowing the distribution of stellar rotational velocities is essential for the understanding stellar evolution. Because we measure the projected rotational speed vsini, we need to solve an ill-posed problem given by a Fredholm integral of the first kind to recover the true rotational velocity distribution. After discretization of the Fredholm integral, we apply the Tikhonov regularization method to obtain directly the probability distribution function for stellar rotational velocities. We propose a simple and straightforward procedure to determine the Tikhonov parameter. We applied Monte Carlo simulations to prove that Tikhonov method is a consistent estimator and asymptotically unbiased. This method is applied to a sample of cluster stars. We obtain confidences intervals using bootstrap method. Our results are in good agreement with the one obtained using the Lucy method, in recovering the probability density distribution of rotational velocities. Furthermore, Lucy estimation lies inside our confidence interval. Tikhonov regularization is a very robust method that deconvolve the rotational velocity probability density function from a sample of vsini data straightforward without needing any convergence criteria.

astro-ph.IM↗
arXiv2016

Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73

We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the LMC B[e] supergiant LHA 120-S 73. High-resolution optical and near-IR spectroscopic data were obtained over a period of 16 and 7 years, respectively. The spectra cover the diagnostic emission lines from [CaII] and [OI], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A low-resolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk. All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly non-monotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The mid-infrared spectrum displays features of oxygen- and carbon-rich grain species, which indicates a long-lived, stable dusty disk. We cannot confirm the previously reported high value for the stellar rotation velocity. The line profile of HeI 5876 A is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible. The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.

astro-ph.SR↗
arXiv2015

Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478

Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.

astro-ph.SR↗
arXiv2014

Line-Driven Winds Revisited in the Context of Be Stars: $\Omega$-slow Solutions with High $k$ Values

The standard, or fast, solutions of m-CAK line-driven wind theory cannot account for slowly outflowing disks like the ones that surround Be stars. It has been previously shown that there exists another family of solutions --- the $\Omega$-slow solutions --- that is characterized by much slower terminal velocities and higher mass-loss rates. We have solved the one-dimensional m-CAK hydrodynamical equation of rotating radiation-driven winds for this latter solution, starting from standard values of the line force parameters ($\alpha$, $k$, and $\delta$), and then systematically varying the values of $\alpha$ and $k$. Terminal velocities and mass-loss rates that are in good agreement with those found in Be stars are obtained from the solutions with lower $\alpha$ and higher $k$ values. Furthermore, the equatorial densities of such solutions are comparable to those that are typically assumed in ad hoc models. For very high values of $k$, we find that the wind solutions exhibit a new kind of behavior.

astro-ph.SR↗
arXiv2012

The sudden appearance of CO emission in LHA 115-S 65

Molecular emission has been detected in several Magellanic Cloud B[e] supergiants. In this Letter, we report on the detection of CO band head emission in the B[e] supergiant LHA 115-S 65, and present a K-band near-infrared spectrum obtained with the Spectrograph for INtegral Field Observation in the Near-Infrared (SINFONI; R=4500) on the ESO VLT UT4 telescope. The observed molecular band head emission in S 65 is quite surprising in light of a previous non-detection by McGregor et al. 1989, as well as a high resolution (R=50000) Gemini/Phoenix spectrum of this star taken nine months earlier showing no emission. Based on analysis of the optical spectrum by Kraus et al. 2010, we suspect that the sudden appearance of molecular emission could be due to density build up in an outflowing viscous disk, as seen for Be stars. This new discovery, combined with variability in two other similar evolved massive stars, indicates an evolutionary link between B[e] supergiants and LBVs.

astro-ph.SR↗
arXiv2012

High-Resolution Near-Infrared Polarimetry of a Circumstellar Disk around UX Tau A

We present H-band polarimetric imagery of UX Tau A taken with HiCIAO/AO188 on the Subaru Telescope. UX Tau A has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0.15 (21 AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UX Tau A which extends to 120 AU, at a spatial resolution of 0.1 (14 AU). It is inclined by 46 \pm 2 degree as the west side is nearest. Although SED modeling and sub-millimeter imagery suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25 - 30 AU, we detect no evidence of a gap at the limit of our inner working angle (23 AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66 %) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh scattering nor Mie scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with the radii of 30 micron meter is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations and have grown in the circumstellar disk of UX Tau A.

astro-ph.SR↗
arXiv2012

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory

We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r~46AU, our observations reveal the presence of scattered light components as close as 0.2" (~28AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0.5" (~70AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h~0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

astro-ph.EP↗
arXiv2011

Characterising the magnetic fields of the Herbig Ae/Be stars HD97048, HD150193, HD176386, and MWC480

Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail. Our results for the three Herbig Ae/Be stars HD97048, HD150193, and HD176386 are based on multi-epoch low-resolution spectra obtained in spectropolarimetric mode with FORS2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS2 low-resolution observations of the Herbig Ae/Be stars HD97048, HD150193, and HD176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC480 were measured using the moment technique where wavelength shifts between right- and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field . Our search for periodicities resulted in P=0.693d for HD97048, P=1.317d for HD150193, and P=0.899d for HD176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength B_d of several hundred Gauss. The longitudinal magnetic-field measurements of MWC480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS1. A weak magnetic field was detected in the circumstellar components of the Ca II H&K lines and the Na I D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment.

astro-ph.SR↗
arXiv2011

Slow Radiation-Driven Wind Solutions of A-Type Supergiants

The theory of radiation-driven winds succeeded in describing terminal velocities and mass loss rates of massive stars. However, for A-type supergiants the standard m-CAK solution predicts values of mass loss and terminal velocity higher than the observed values. Based on the existence of a slow wind solution in fast rotating massive stars, we explore numerically the parameter space of radiation-driven flows to search for new wind solutions in slowly rotating stars, that could explain the origin of these discrepancies. We solve the 1-D hydrodynamical equation of rotating radiation-driven winds at different stellar latitudes and explore the influence of ionization's changes throughout the wind in the velocity profile. We have found that for particular sets of stellar and line-force parameters, a new slow solution exists over the entire star when the rotational speed is slow or even zero. In the case of slow rotating A-type supergiant stars the presence of this novel slow solution at all latitudes leads to mass losses and wind terminal velocities which are in agreement with the observed values. The theoretical Wind Momentum-Luminosity Relationship derived with these slow solutions shows very good agreement with the empirical relationship. In addition, the ratio between the terminal and escape velocities, which provides a simple way to predict stellar wind energy and momentum input into the interstellar medium, is also properly traced.

astro-ph.SR↗
arXiv2010

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite of the importance of magnetic fields for the full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied over the rotation cycle until now. One reason for the paucity of such observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD101412 with a strong surface magnetic field became in the last years one of the most studied targets among the Herbig Ae/Be stars. A few months ago we obtained multi-epoch polarimetric spectra of this star with FORS2 to search for a rotation period and to constrain the geometry of the magnetic field. We measured longitudinal magnetic fields on 13 different epochs distributed over 62 days. These new measurements together with our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. The search of the rotation period resulted in P=42.076+-0.01d. According to near-infrared imaging studies the star is observed nearly edge-on. The star exhibits a single-wave variation of the longitudinal magnetic field during the stellar rotation cycle. These observations are usually considered as evidence for a dominant dipolar contribution to the magnetic field topology.

astro-ph.SR↗
arXiv2009

Magnetic survey of emission line B-type stars with FORS1 at the VLT

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star upsilon Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ~1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD62367, mu Cen, o Aqr, and epsilon Tuc. The strongest longitudinal magnetic field, =117+-38G, was detected in the Be star HD62367. Among the Be stars studied with time series, one Be star, lambda Eri, displays cyclic variability of the magnetic field with a period of 21.12min. The binary star upsilon Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field =-102+-10G at MJD54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members.

astro-ph.SR↗
arXiv2009

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks

Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous magnetic field detections. The largest longitudinal magnetic field, = -454+-42G, was detected in the Herbig Ae/Be star HD101412 using hydrogen lines. No field detection at a significance level of 3sigma was achieved in stars with debris disks. Our study does not indicate any correlation of the strength of the longitudinal magnetic field with disk orientation, disk geometry, or the presence of a companion. We also do not see any simple dependence on the mass-accretion rate. However, it is likely that the range of observed field values qualitatively supports the expectations from magnetospheric accretion models giving support for dipole-like field geometries. Both the magnetic field strength and the X-ray emission show hints for a decline with age in the range of ~2-14Myrs probed by our sample supporting a dynamo mechanism that decays with age. However, our study of rotation does not show any obvious trend of the strength of the longitudinal magnetic field with rotation period. Furthermore, the stars seem to obey the universal power-law relation between magnetic flux and X-ray luminosity established for the Sun and main-sequence active dwarf stars.

astro-ph.SR↗
arXiv2009

Magnetic fields in classical Be stars

We report the results of our study of magnetic fields in a sample of Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. The detected magnetic fields are rather weak, not stronger than ~150G. A few classical Be stars display cyclic variability of the magnetic field with periods of tens of minutes.

astro-ph.SR↗
arXiv2005

Outflowing disk formation in B[e] supergiants due to rotation and bi--stability in radiation driven winds

The effects of rapid rotation and bi-stability upon the density contrast between the equatorial and polar directions of a B[e] supergiant are re-investigated. Based upon a new slow solution for different high rotational radiation driven winds (Curé 2004) and the fact that bi--stability allows a change in the line--force parameters ($α$, $k$, and $δ$), the equatorial densities are about $10^2$--$10^4$ times higher than the polar ones. These values are in qualitative agreement with the observations.

astro-ph↗