Intermediate-mass Main-sequence Stars Research Articles

The AstraLux-TESS high spatial resolution imaging survey. Search for stellar companions of 215 planet candidates from TESS

Chance-aligned sources or blended companions can cause false positives in planetary transit detections or simply bias the determination of the candidate properties. In the era of high-precision space-based photometers, the need for high spatial resolution images has been demonstrated to be critical for validating and confirming transit signals. This already applied to the Kepler mission, is now applicable to the TESS survey, and will be critical for the PLATO mission. In this paper we present the results of the AstraLux-TESS survey, a catalog of high spatial resolution images obtained with the AstraLux instrument at the Calar Alto observatory (Almer\'ia, Spain) in the context of the TESS Follow-up Observing Program. We used the lucky imaging technique to obtain high spatial resolution images from planet candidate hosts included mostly in two relevant regimes: exoplanet candidates belonging to the level one requirement of the TESS mission (planets with radii $R<4 R_ oplus $) and TESS planet candidates around intermediate-mass main-sequence stars. Among the 185 planet host candidate stars observed, we found 13 (7<!PCT!>) to be accompanied by additional sources within a separation of 2.2 arcsec . Among them, six are not associated with sources in the DR3 catalog, thus contaminating the TESS light curve. Even if no contaminants have been detected, we can provide upper limits and probabilities to the possible existence of field contaminants through the sensitivity limits of our images. Among the isolated hosts, we can discard hazardous companions (bright enough to mimic a planetary transit signals) with an accuracy below 1<!PCT!> for all their planets. The results from this catalog are key to the statistical validation of small planets (prime targets of the TESS mission) and planets around intermediate-mass stars in the main sequence. These two populations of planets are difficult to confirm with the radial velocity technique because of the shallow amplitude of small planets and the high rotational velocities and low number of available spectral lines in the intermediate stellar mass regime. Our results also demonstrate the importance of this type of follow-up observation for future transit missions such as PLATO, even in the era.

Semi-analytical Expression of G-mode Period Spacing: The Case of Brunt–Väisälä Frequency with Not a Jump but a Ramp

To decipher complex patterns of gravity-mode period spacings observed for intermediate-mass main-sequence stars is an important step toward a better understanding of the structure and dynamics in the deep radiative region of the stars. In this study, we apply the JWKB approximation to derive a semi-analytical expression of the g-mode period spacing pattern, for which the gradient in the Brunt–Väisälä frequency is taken into account. The formulation includes the term P −1 B ⋆, where P and B ⋆ represent the g-mode period and degree of the structural variation, the latter of which especially is related to the steepness of the gradient of the Brunt–Väisälä frequency. Tests with one-dimensional stellar models show that the semi-analytical expression derived in this study is useful for inferring the degree of the structural variation B ⋆ with an accuracy of ∼10 % in the case of relatively massive intermediate-mass models with the mass M larger than 3 M ⊙. The newly formulated expression will possibly allow us to put further constraints on, e.g., mixing processes inside intermediate-mass main-sequence g-mode pulsators such as β Cep, SPB, and γ Dor stars that have been principal targets in asteroseismology.

Stellar evolution models with overshooting based on 3-equation non-local theories

Context.Mixing by convective overshooting has long been suggested to play an important role in the amount of hydrogen available for nuclear burning in convective cores of stars. The best way to model this effect is still debated.Aims.We suggest an improved model for the computation of the dissipation rate of turbulent kinetic energy which can be used in non-local models of stellar convection and can readily be implemented and self-consistently used in 1D stellar evolution calculations.Methods.We review the physics underlying various models to compute the dissipation rate of turbulent kinetic energy,ϵ, in local models of convection in stellar astrophysics and particularly in non-local ones. The different contributions to the dissipation rate and their dependence on local stratification and on non-local transport are analysed and a new method to account for at least some of these physical mechanisms is suggested.Results.We show how the new approach influences predictions of stellar models of intermediate-mass main-sequence stars and how these changes differ from other modifications of the non-local convection model that focus on the ratio of horizontal to vertical (turbulent) kinetic energy.Conclusions.The new model is shown to allow for a physically more complete description of convective overshooting and mixing in massive stars. Dissipation by buoyancy waves is found to be a key ingredient which has to be accounted for in non-local models of turbulent convection.

2D modelling of pulsating stars with rapid rotation

Rapid stellar rotation is an important phenomenon in stellar physics, particularly for massive and intermediate mass main-sequence stars. This affects all aspects of the star’s physics including its structure, evolution, and pulsations, and makes it necessary to use 2D numerical approaches rather than the 1D approaches typically used. In this contribution, we will review 2D numerical methods for modelling and interpreting pulsation modes in rapidly rotating stars. We will start by deriving the pulsation equations, both in an adiabatic and non-adiabatic setting, then provide a description of the 2D numerical implementation. We will then explain approximate implementations of the effects of rotation, namely first, second, and third order perturbative approaches, as well as the traditional approximation. This will then be followed by a description on how to calculate disk-integrated mode visibilities in various photometric bands, and how to apply this to mode identification in rapid rotators. Finally, we will review some of the recent works that interpret the pulsation spectra of various stars as viewed in either a single photometric band or in multiple bands, and including supplementary constraints from interferometry and spectroscopy.

Magnetized Oscillatory Double-diffusive Convection

We study the properties of oscillatory double-diffusive convection (ODDC) in the presence of a uniform vertical background magnetic field. ODDC takes place in stellar regions that are unstable according to the Schwarzschild criterion and stable according to the Ledoux criterion (sometimes called semiconvective regions), which are often predicted to reside just outside the core of intermediate-mass main-sequence stars. Previous hydrodynamic studies of ODDC have shown that the basic instability saturates into a state of weak wave-like convection, but that a secondary instability can sometimes transform it into a state of layered convection, where layers then rapidly merge and grow until the entire region is fully convective. We find that magnetized ODDC has very similar properties overall, with some important quantitative differences. A linear stability analysis reveals that the fastest-growing modes are unaffected by the field, but that other modes are. Numerically, the magnetic field is seen to influence the saturation of the basic instability, overall reducing the turbulent fluxes of temperature and composition. This in turn affects layer formation, usually delaying it, and occasionally suppressing it entirely for sufficiently strong fields. Further work will be needed, however, to determine the field strength above which layer formation is actually suppressed in stars. Potential observational implications are briefly discussed.

Mass–effective temperature–surface gravity relation for intermediate-mass main-sequence stars

Context. In this work a mass–effective temperature–surface gravity relation (MTGR) is developed for main-sequence stars in the range 6400 K ≤ Teff ≤ 20 000 K with log g ≳ 3.44. The MTGR allows the simple estimation of the masses of stars from their effective temperatures and surface gravities. It can be used for solar metallicity and can be rescaled for any metallicity within −1.0 ≤ [Fe/H] ≤ 0.7. The effect of α-enhanced compositions can also be considered with the help of correction terms. Aims. It is aimed to develop an MTGR that can estimate the masses of main-sequence stars from their atmospheric parameters. One advantage of an MTGR over the classical mass–luminosity relations is that its mass estimation is based on parameters that can be obtained by purely spectroscopic methods, and, therefore, the interstellar extinction or reddening do not have to be known. The use of surface gravity (g) also relates an MTGR with stellar evolution and provides a more reliable mass estimation. Methods. A synthetical MTGR is obtained from theoretical isochrones using a Levenberg-Marquardt χ2 minimization algorithm. The validity of the MTGR is then checked by testing over 278 binary components with precise absolute masses. Results. Very good agreement has been obtained between the absolute masses of 278 binary star components and their masses estimated from the MTGR. A mathematical expression is also given to calculate the propagated uncertainties of the MTGR masses. Conclusions. For the typical uncertainties in atmospheric parameters and metallicity (i.e., ±2.8% for Teff, ±0.1 dex for log g and ±0.15 dex for [Fe/H]) the typical uncertainties in the masses estimated from the MTGR mostly remain around 5–9%. The fact that this uncertainty level is only on average about three times as large as that of the absolute masses indicates that the MTGR is a very powerful tool for stellar mass estimation. A computer code, mtgr.pro, written in GDL or IDL is also provided for the relation.

Very regular high-frequency pulsation modes in young intermediate-mass stars.

Asteroseismology probes the internal structures of stars by using their natural pulsation frequencies1. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars2, red giants3, high-mass stars4 and white dwarfs5. However, a large group of pulsating stars of intermediate mass-the so-called δ Scuti stars-have rich pulsation spectra for which systematic mode identification has not hitherto been possible6,7. This arises because only a seemingly random subset of possible modes are excited and because rapid rotation tends to spoil regular patterns8-10. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, which enables definitive mode identification. The space motions of some of these stars indicate that they are members of known associations of young stars, as confirmed by modelling of their pulsation spectra.

Magnetic geometry and surface differential rotation of the bright Am star Alhena A

ABSTRACT Alhena A (γ Gem A) is a bright Am star, with the strongest disc-integrated magnetic field strength reported so far for an Am star. Its spectrum exhibits standard circularly polarized Zeeman signatures, contrary to all previously studied Am stars that display abnormal signatures dominated by a single-signed lobe. We present here the result of follow-up observations of Alhena, using very high signal-to-noise spectropolarimetric data obtained over 25 observing nights with NARVAL at Télescope Bernard Lyot, in the frame of the BRITE (BRIght Target Explorer) spectropolarimetric survey. We confirm that Alhena A is magnetic and we determine its surface magnetic properties using different methods. Inclined dipole models are used to reproduce the longitudinal field measurements, as well as the Stokes V line profiles themselves. In both cases, the model is consistent with a polar field strength of ∼30 G. This is confirmed by a Zeeman-Doppler Imaging (ZDI) model, which also unveils smaller scale magnetic structures. A rotational period of 8.975 d was identified using intensity line profile variations. The ZDI inversion suggests that the surface magnetic field is sheared by differential rotation, with a difference in rotation rate between high and low latitudes at about 15 per cent of the solar value. This result challenges theories of the development of surface differential rotation in intermediate mass main-sequence stars.

Magnetic fields of intermediate-mass T Tauri stars

Context. The origin of the fossil magnetic fields detected in 5 to 10% of intermediate-mass main sequence stars is still highly debated.Aims. We want to bring observational constraints to a large population of intermediate-mass pre-main sequence (PMS) stars in order to test the theory that convective-dynamo fields generated during the PMS phases of stellar evolution can occasionally relax into fossil fields on the main sequence.Methods. Using distance estimations, photometric measurements, and spectropolarimetric data from HARPSpol and ESPaDOnS of 38 intermediate-mass PMS stars, we determined fundamental stellar parameters (Teff,Landvsini) and measured surface magnetic field characteristics (including detection limits for non-detections, and longitudinal fields and basic topologies for positive detections). Using PMS evolutionary models, we determined the mass, radius, and internal structure of these stars. We compared different PMS models to check that our determinations were not model-dependant. We then compared the magnetic characteristics of our sample accounting for their stellar parameters and internal structures.Results. We detect magnetic fields in about half of our sample. About 90% of the magnetic stars have outer convective envelopes larger than ∼25% of the stellar radii, and heavier than ∼2% of the stellar mass. Going to higher mass, we find that the magnetic incidence in intermediate-mass stars drops very quickly, within a timescale on the order of few times 0.1 Myr. Finally, we propose that intermediate-mass T Tauri stars with large convective envelopes, close to the fully convective limit, have complex fields and that their dipole component strengths may decrease as the sizes of their convective envelopes decrease, similar to lower-mass T Tauri stars.

Sensitivity of gravito-inertial modes to differential rotation in intermediate-mass main-sequence stars

Context. While rotation has a major impact on stellar structure and evolution, its effects are not well understood. Thanks to high-quality and long-time base photometric observations obtained with recent space missions, we are now able to study stellar rotation more precisely.Aims. We aim to constrain radial differential rotation profiles inγDoradus (γDor) stars, and to develop new theoretical seismic diagnosis for such stars with rapid and potentially non-uniform rotation.Methods. We have derived a new asymptotic description which accounts for the impact of weak differential near-core rotation on gravity-mode period spacings. The theoretical predictions are illustrated from pulsation computations with the code GYRE and compared with observations ofγDor stars. When possible, we also derived the surface rotation rates in these stars by detecting and analysing signatures of rotational modulation, and computed the core-to-surface rotation ratios.Results. Stellar rotation must be strongly differential before its effects on period spacing patterns can be detected, unless multiple period spacing patterns can be compared. Six stars in our sample exhibit a single unexplained period spacing pattern of retrograde modes. We hypothesise that these are Yanai modes. Finally, we find signatures of rotational spot modulation in the photometric data of eight targets.Conclusions. If only one period spacing pattern is detected and analysed for a star, it is difficult to detect differential rotation. A rigidly rotating model will often provide the best solution. Differential rotation can only be detected when multiple period spacing patterns have been found for a single star or its surface rotation rate is known as well. This is the case for eight of the stars in our sample, revealing surface-to-core rotation ratios between 0.95 and 1.05.

2-D and 3-D models of convective turbulence and oscillations in intermediate-mass main-sequence stars

AbstractWe present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid γ Dor/δ Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations of 1-2 M⊙ stars gained by multidimensional modeling.

Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049

Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of ~30 kG, one of the strongest known for any non-degenerate star. We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field. We refined the rotational period of HD 75049 to Prot=4.048267+/-0.000036 d. We also derived basic stellar parameters, Teff=10250+/-250 K and logg=4.3+/-0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5-1.4 dex, which is low compared with many other Ap stars. Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole. The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediate-mass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths.

Enhancement of surface helium abundance in intermediate-mass main-sequence stars

The evolution of rapidly rotating 8, 4, and 2 M⊙ main-sequence stars is considered together with hydrodynamical transfer in their interiors. The conditions under which turbulent erosion, semiconvection, and shear turbulence lead to partial mixing of the matter in the radiative envelope and central regions of the stars are determined. The enhancement of the surface helium abundance with time depends on both the intensity of partial mixing in their interiors and mass loss by the stellar wind. The ratio of the number densities of helium and hydrogen at the surface can rise by the end of main-sequence stage by ∼30% for a 8 M⊙ star and ∼10−20% for a 4 M⊙ star, depending on the mass-loss rate. Partial mixing of the matter in the radiative envelope and in the central region of the star can provide an explanation for the observed enhancement of the atmospheric helium abundances of early B stars toward the end of their main-sequence evolution. The enhancement of the surface helium abundance in a 2 M⊙ star is so small that it cannot be detected, and is appreciably lower than the enhancement beneath the surface.

The dichotomy between strong and ultra-weak magnetic fields among intermediate-mass stars

AbstractUntil recently, the detection of magnetic fields at the surface of intermediate-mass main-sequence stars has been limited to Ap/Bp stars, a class of chemically peculiar stars. This class represents no more than 5-10% of the stars in this mass range. This small fraction is not explained by the fossil field paradigm that describes the Ap/Bp type magnetism as a remnant of an early phase of the star-life. Also, the limitation of the field measurements to a small and special group of stars is obviously a problem to study the effect of the magnetic fields on the stellar evolution of a typical intermediate-mass star.Thanks to the improved sensitivity of a new generation of spectropolarimeters, a lower bound to the magnetic fields of Ap/Bp stars, a two orders of magnitude desert in the longitudinal magnetic field and a new type of sub-gauss magnetism first discovered on Vega have been identified. These advances provide new clues to understand the origin of intermediate-mass magnetism as well as its influence on stellar evolution. In particular, a scenario has been proposed whereby the magnetic dichotomy between Ap/Bp and Vega-like magnetism originate from the bifurcation between stable and unstable large scale magnetic configurations in differentially rotating stars. In this paper, we review these recent observational findings and discuss this scenario.

OBSERVATIONS OF MAIN-SEQUENCE STARS AND LIMITS ON EXOZODICAL DUST WITH NULLING INTERFEROMETRY

We present nulling interferometric observations of six nearby main-sequence stars (α CrB, α Lyr, β Leo, γ Ser, Eri, and ζ Lep). None of the stars show evidence for a positive detection of warm debris in the habitable zone of the systems. Using a scaled up model of solar zodiacal emission, the 3σ upper limits on dust density range from 220 to 104 Zody (1 Zody = the density of our own solar zodiacal cloud) depending on the particular star, corresponding to mass limits of 10–7 to 10–5 M ⊕ of micron-sized dust. These limits contrast with the presence of dust at greater separations from the star, implying a clearing in dust in the inner system. This suggests that the inner circumstellar environment around nearby intermediate-mass main-sequence stars more than several tens of millions of years old are generally cleared of dusty debris. Finally, the well studied nature of the debris disks around Vega, Eridani, and ζ Leporis allows us to place these 10 μm nulling observations in the context of previous studies to determine the physical processes responsible for shaping the debris disk in these particular systems.

Spectroscopic and photometric observations of selected Algol-type binaries – I. V1665 Aquilae and AG Arietis

We present the first radial velocities and ground-based photometric observations of V1665 Aql and AG Ari. These double-lined eclipsing binaries have eccentric orbits with eccentricities of 0.24 and 0.09, respectively. New UBV photometric data and radial velocities were analysed using a modified version of the Wilson-Devinney program for the parameters of the systems. Good phase coverage and relatively small scatter both in the light and in the radial-velocity curves meant that the eccentricity and the argument of periastron as well as orbital and stellar parameters could be determined. Our solutions indicate that AG Ari includes a third light with a considerable amount of 5.6 per cent in U, 6.4 per cent in B, and 9 per cent in V. By comparing its light contribution with the total light we classified it as an F-type star. The components in each system are of very nearly equal mass, temperature, and luminosity. Their absolute parameters were compared with those of stars belonging to well-studied detached eclipsing binaries. The components lie well inside the main-sequence band. In addition, the apsidal motion rates of the system have been determined from an analysis of all the photometric observations obtained by space missions and by us. We determined the preliminary apsidal motion periods of the systems to be 449 ± 34 yr for V1665 Aql and 155 ± 6 yr for AG Ari. Because well-detached eclipsing binaries are optimal distance indicators, we estimated the distances to V1665 Aql and AG Ari to be 477 ± 40 and 292 ± 18 pc from the determined absolute visual magnitudes and interstellar extinction. A comparison with evolutionary tracks indicates that the primary component of V1665 Aql has an age of about 1.6 x 10 8 yr and is near the main-sequence terminal age. The components of AG Ari are intermediate-mass main-sequence stars with an age of about 5.6 × 10 8 yr.

Star cluster ecology - VII. The evolution of young dense star clusters containing primordial binaries

We study the first ∼100 Myr of the evolution of isolated star clusters initially containing 144 179 stars, including 13 107 (10 per cent) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high-precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (≤100 Myr) star clusters in the large Magellanic Cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (∼10 per cent). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as a = - 1.8 in the central part of the cluster (where the initial Salpeter mass function had a = -2.35). About 6-12 per cent of the neutron stars were retained in our simulations; the fraction of retained black holes is 40-70 per cent. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an X-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find ∼11 potential X-ray binaries containing a black hole. Binaries consisting of two white dwarfs are quite common, but few (20-30) are sufficiently close that they will merge within a Hubble time due to the emission of gravitational radiation. Clusters with shorter relaxation times tend to produce fewer merging white dwarf binaries. The white dwarf binaries that do merge are all sufficiently massive to produce a Type Ia supernova. The densest cluster produces about twice as many blue stragglers as a field population containing the same number of binaries, and these blue stragglers are more massive, bluer and brighter than in less dense clusters.

Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

Context.The X-ray emission from B-type main-sequence stars is a longstanding mystery in stellar coronal research. Since there is no theory at hand that explains intrinsic X-ray emission from intermediate-mass main-sequence stars, the observations have often been interpreted in terms of (unknown) late-type magnetically active companion stars.

Magnetic properties of intermediate-mass stars

Magnetic fields play an important role in producing and modifying the photospheric chemical peculiarities of intermediate-mass main sequence stars. This article discusses the basic theory and methods of measurement used to detect and characterise stellar magnetic fields, and reviews our current knowledge of selected characteristics of magnetic fields in intermediate-mass stars.

Infrared observations of NGC 3603

We have performed deep K s -band observations of the starburst region NGC 3603 with the infrared camera ISAAC mounted on the VLT Antu. The total area covered by our data stretches from the NGC 3603 starburst cluster towards the south up to a maximal distance of about 370$\hbox{$^{\prime\prime}$}$ (~12 pc). This enables us to reconsider and redetermine the radial extent of the cluster which is found to be about 150$\hbox{$^{\prime\prime}$}$ ± 15$\hbox{$^{\prime\prime}$}$ (~5 pc), exceeding previous estimates by a factor of 2.5. King model fits are used to disentangle the count statistics of cluster stars from those of field stars. With knowledge of the cluster radius we then construct and analyze the K s -band luminosity function (KLF) of the NGC 3603 starburst cluster. The KLF for cluster radii >30$\hbox{$^{\prime\prime}$}$ is rising down to the completeness limit of our study at K s ~ 17$\hbox{m}$–17$\hbox{m}\!\!\!.\,$5, corresponding to 1 Myr old cluster members of $\cal M$ ~ 0.5 $\cal M\hbox{$_{\odot}$}$. For the range of intermediate luminosity stars we obtain a KLF slope of α = 0.35 ± 0.02 which is consistent with a Miller-Scalo type initial mass function (IMF) of a ~1 Myr old stellar cluster if one assumes a power-law index of β ~ 2 for the mass-luminosity relation of both intermediate mass main sequence stars and low mass pre-main sequence stars. At the high luminosity end the observed KLF likely flattens ( α ~ 0.2), being consistent with a Salpeter type IMF for NGC 3603's massive main sequence stars.