Early-type Main-sequence Stars Research Articles

TESS search for substellar companions through pulsation timing of δ Scuti stars

Early-type main-sequence pulsating stars such as δ Scuti variables are one of the least explored class of targets in the search for exoplanets. Pulsation timing (PT) is an alternative technique to the most effective search methods. It exploits the light-travel-time effect (LTTE) to infer the presence of additional massive bodies around a pulsating star by measuring a periodic phase modulation of its signal. PT has been extremely successful in discovering and characterizing stellar binaries when it was applied to high-precision light curves over long temporal baselines, such as those delivered by the Kepler mission. In favorable conditions, the sensitivity of PT can reach the planetary-mass regime; one such candidate has already been claimed. The advent of TESS, with its nearly full-sky coverage and the availability of full-frame images, opens a great opportunity to expand this field of research. In this work, we present a pilot study aimed to understand the potential of PT as applied to TESS data, which are considerably different from Kepler data in terms of photometric noise, sampling cadence, and temporal baseline. We focused on the most favorable class of δ Scuti, that is, those showing strong pulsations and very simple frequency spectra. After the development of a customized pipeline, we were able to detect candidate companions for two targets in the (sub-)stellar mass regime: Chang 134 (43 ± 5 Mjup, P ≃ 82 days) and V393 Car (≳100 Mjup, P ≳ 700 days). Our results also highlight the limiting factors of this technique and the importance of an accurate absolute time calibration for future missions such as PLATO.

Rapidly rotating stars and their transiting planets: KELT-17b, KELT-19Ab, and KELT-21b in the CHEOPS and TESS era

ABSTRACT Rapidly rotating early-type main-sequence stars with transiting planets are interesting in many aspects. Unfortunately, several astrophysical effects in such systems are not well understood yet. Therefore, we performed a photometric mini-survey of three rapidly rotating stars with transiting planets, namely KELT-17b, KELT-19Ab, and KELT-21b, using the Characterising Exoplanets Satellite (CHEOPS), complemented with Transiting Exoplanet Survey Satellite (TESS) data, and spectroscopic data. We aimed at investigating the spin-orbit misalignment and its photometrical signs, therefore the high-quality light curves of the selected objects were tested for transit asymmetry, transit duration variations, and orbital precession. In addition, we performed transit time variation analyses, obtained new stellar parameters, and refined the system parameters. For KELT-17b and KELT-19Ab, we obtained significantly smaller planet radius as found before. The gravity-darkening effect is very small compared to the precision of CHEOPS data. We can report only on a tentative detection of the stellar inclination of KELT-21, which is about 60 deg. In KELT-17b and KELT-19Ab, we were able to exclude long-term transit duration variations causing orbital precession. The shorter transit duration of KELT-19Ab compared to the discovery paper is probably a consequence of a smaller planet radius. KELT-21b is promising from this viewpoint, but further precise observations are needed. We did not find any convincing evidence for additional objects in the systems.

Non-linear excitation of low-frequency modes by overstable convective modes in rotating stars

ABSTRACT We discuss non-linear excitation and amplitude saturation of g-, r, and overstable convective (OsC) modes in early-type main sequence stars, taking account of the effects of three-mode couplings on amplitude evolutions. OsC modes are rotationally stabilized convective modes in the convective core and they resonantly excite low-frequency g-modes to obtain large amplitudes in the envelope when the rotation rate of the core is larger than critical rates. We use, for a network of three-mode couplings, amplitude equations governing the time evolution of the mode amplitudes where each of three-mode couplings is assumed to occur between two stable modes and one unstable mode. Assuming that the unstable modes in the couplings are OsC modes in the core and the stable modes are g- and r-modes in the envelope, we integrate the amplitude equations to see how the g- and r-modes are non-linearly excited by the OsC modes and whether or not the amplitude evolutions tend toward a state of finite amplitudes. We find that the non-linear three-mode couplings do excite low-frequency g- and r-modes but they are not necessarily effective to achieve amplitude saturation since the three-mode couplings between the OsC modes with large growth rates and g- and r-modes with small-damping rates tend to destabilize amplitude evolutions.

Convective Penetration in Early-type Stars

Observations indicate that the convective cores of stars must ingest a substantial amount of material from the overlying radiative zone, but the extent of this mixing and the mechanism that causes it remain uncertain. Recently, Anders et al. developed a theory of convective penetration and calibrated it with 3D numerical hydrodynamics simulations. Here we employ that theory to predict the extent of convective boundary mixing (CBM) in early-type main-sequence stars. We find that convective penetration produces enough mixing to explain core masses inferred from asteroseismology and eclipsing binary studies, and matches observed trends in mass and age. While there are remaining uncertainties in the theory, this agreement suggests that most CBM in early-type main-sequence stars arises from convective penetration. Finally, we provide a fitting formula for the extent of core convective penetration for main-sequence stars in the mass range from 1.1–60 M ⊙.

Exploring the Stellar Rotation of Early-type Stars in the LAMOST Medium-resolution Survey. II. Statistics

Abstract Angular momentum is a key property regulating star formation and evolution. However, the physics driving the distribution of the stellar rotation rates of early-type main-sequence stars is as yet poorly understood. Using our catalog of 40,034 early-type stars with homogeneous v sin i parameters, we review the statistical properties of their stellar rotation rates. We discuss the importance of possible contaminants, including binaries and chemically peculiar stars. Upon correction for projection effects and rectification of the error distribution, we derive the distributions of our sample’s equatorial rotation velocities, which show a clear dependence on stellar mass. Stars with masses less than 2.5 M ⊙ exhibit a unimodal distribution, with the peak velocity ratio increasing as stellar mass increases. A bimodal rotation distribution, composed of two branches of slowly and rapidly rotating stars, emerges for more massive stars (M > 2.5 M ⊙). For stars more massive than 3.0 M ⊙, the gap between the bifurcated branches becomes prominent. For the first time, we find that metal-poor ([M/H] < −0.2 dex) stars only exhibit a single branch of slow rotators, while metal-rich ([M/H] > 0.2 dex) stars clearly show two branches. The difference could be attributed to unexpectedly high spin-down rates and/or in part strong magnetic fields in the metal-poor subsample.

Mapping the core of the Tarantula Nebula with VLT-MUSE

We present the spectroscopic analysis of 333 OB-type stars extracted from VLT-MUSE observations of the central 30 × 30 pc of NGC 2070 in the Tarantula Nebula on the Large Magellanic Cloud, the majority of which are analysed for the first time. The distribution of stars in the spectroscopic Hertzsprung-Russell diagram (sHRD) shows 281 stars in the main sequence. We find two groups in the main sequence, with estimated ages of 2.1 ± 0.8 and 6.2 ± 2 Myr. A subgroup of 52 stars is apparently beyond the main sequence phase, which we consider to be due to emission-type objects and/or significant nebular contamination affecting the analysis. As in previous studies, stellar masses derived from the sHRD are systematically larger than those obtained from the conventional HRD, with the differences being largest for the most massive stars. Additionally, we do not find any trend between the estimated projected rotational velocity and evolution in the sHRD. The projected rotational velocity distribution presents a tail of fast rotators that resembles findings in the wider population of 30 Doradus. We use published spectral types to calibrate the He Iλ4921/He IIλ5411 equivalent-width ratio as a classification diagnostic for early-type main sequence stars when the classical blue-visible region is not observed. Our model-atmosphere analyses demonstrate that the resulting calibration is well correlated with effective temperature.

Rotating convective core excites non-radial pulsations to cause rotational modulations in early-type stars

ABSTRACT We discuss low-frequency g modes excited by resonant couplings with weakly unstable oscillatory convective modes in the rotating convective core in early-type main-sequence stars. Our non-adiabatic pulsation analyses including the effect of Coriolis force for $2\, \mathrm{ M}_\odot$ main-sequence models show that if the convective core rotates slightly faster than the surrounding radiative layers, g modes in the radiative envelope are excited by a resonance coupling. The frequency of the excited g mode in the inertial frame is close to |mΩc| with m and Ωc being the azimuthal order of the g mode and the rotation frequency of the convective core, respectively. These g-mode frequencies are consistent with those of photometric rotational modulations and harmonics observed in many early-type main-sequence stars. In other words, these g modes provide a non-magnetic explanation for the rotational light modulations detected in many early-type main-sequence stars.

Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34

We report on results obtained with theXMM-Newtonobservation of Feige 34 carried out in April 2018. This is the first spectroscopic X-ray observation of a compact and helium-poor hot subdwarf star. The source was detected at a flux levelfX = 3.4 × 10−14erg cm−2s−1in the energy range 0.2–3 keV, which implies an X-ray-to-bolometric flux ratiofX/fbol ≃ 10−6.5. The source spectrum can be described with the sum of two thermal-plasma components with subsolar abundances at temperatures of ≃0.3 and 1.1 keV. These properties are similar to what is observed in early-type main-sequence stars, where the X-ray emission is attributed to turbulence and shocks in the stellar wind. Therefore, the same phenomenon could explain the X-ray properties of Feige 34. However, it is not possible to reproduce the observed spectrum with a thermal-plasma model if the elemental abundances are fixed at the values obtained from the optical and UV spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are consistent with those expected from a young main-sequence star of late spectral type. Therefore, we discuss the possibility that the observed X-ray emission is due to the companion star of M0 spectral type, whose presence is suggested by the IR excess in the spectral energy distribution of Feige 34.

Microlensing masses via photon bunching

In microlensing of a Galactic star by a brown dwarf or other compact object, the amplified image really consists of two unresolved images with slightly different light-travel times. The difference (of order a microsecond) is GM/c^3 times a dimensionless factor depending on the total magnification. Since magnification is well-measured in microlensing events, a single time-delay measurement would provide the mass of the lens, without degeneracies. The challenge is to find an observable that varies on sub-microsecond time scales. This paper notes that the narrow-band intensity of the unresolved image pair will show photon bunching (the Hanbury Brown and Twiss effect), and argues that the lensed intensity will have an auto-correlation peak at the lensing time delay. The ultrafast photon-counting technology needed for this type of measurement exists, but the photon numbers required to give sufficient signal-to-noise appear infeasible at present. Preliminary estimates suggest time-delayed photon bunching may be measurable for lensed early-type main-sequence stars at 10 kpc, with the help of 30 m-class telescopes.

On the nature of the candidate T-Tauri star V501 Aurigae★

We report new multi-colour photometry and high-resolution spectroscopic observations of the long-period variable V501 Aur, previously considered to be a weak-lined T-Tauri star belonging to the Taurus-Auriga star-forming region. The spectroscopic observations reveal that V501 Aur is a single-lined spectroscopic binary system with a 68.8-day orbital period, a slightly eccentric orbit (e ~ 0.03), and a systemic velocity discrepant from the mean of Taurus-Auriga. The photometry shows quasi-periodic variations on a different, ~55-day timescale that we attribute to rotational modulation by spots. No eclipses are seen. The visible object is a rapidly rotating (vsini ~ 25 km/s) early K star, which along with the rotation period implies it must be large (R > 26.3 Rsun), as suggested also by spectroscopic estimates indicating a low surface gravity. The parallax from the Gaia mission and other independent estimates imply a distance much greater than the Taurus-Auriga region, consistent with the giant interpretation. Taken together, this evidence together with a re-evaluation of the LiI~$\lambda$6707 and H$\alpha$ lines shows that V501 Aur is not a T-Tauri star, but is instead a field binary with a giant primary far behind the Taurus-Auriga star-forming region. The large mass function from the spectroscopic orbit and a comparison with stellar evolution models suggest the secondary may be an early-type main-sequence star.

Energy-dependent orbital phases in NGC 300 X-1

NGC~300 X-1 is a Wolf Rayet + black hole binary that exhibits periodic decreases in X-ray flux. We present two new observations of NGC~300 X-1 from the Chandra X-ray Observatory (totaling $\sim$130 ks) along with ACS imaging data from the Hubble Space Telescope. We observe significant short-term variability in the X-ray emission that is inconsistent with an occultation by the donor star, but is consistent with structure in the outer accretion disk or the wind of the donor star. We simultaneously fit a partially-covered disk blackbody and Comptonized corona model to the eclipse egress and non-eclipsing portions of the X-ray spectrum. We find that the only model parameters that varied between the eclipse egress and non-eclipsing portions of the spectra were the partial covering fraction ($\sim$86% during eclipse egress and $\sim$44% during non-eclipse) and absorbing column ($\sim$12.3$\times10^{22}$ cm$^{-2}$ during eclipse egress, compared to $\sim$1.4$\times10^{22}$ cm$^{-2}$ during non-eclipse). The X-ray spectra are consistent with the movement of the X-ray source through the dense stellar winds of the companion star. From our new HST imaging, we find the WR star within the X-ray error circle, along with additional optical sources including an AGB star and an early-type main sequence star. Finally, we use our egress measurement to rephase previous radial velocity measurements reported in the literature, and find evidence that the velocities are strongly affected by the ionization of the wind by the compact object. Thus, we argue the inferred mass of the black hole may not be reliable.

Extrasolar planets and brown dwarfs around A-F type stars

In the frame of the search for extrasolar planets and brown dwarfs around early-type main-sequence stars, we present the detection of a giant planet around the young F-type star HD113337. We estimated the age of the system to be 150 +100/-50 Myr. Interestingly, an IR excess attributed to a cold debris disk was previously detected on this star. The SOPHIE spectrograph on the 1.93m telescope at Observatoire de Haute-Provence was used to obtain ~300 spectra over 6 years. We used our SAFIR tool, dedicated to the spectra analysis of A and F stars, to derive the radial velocity variations. The data reveal a 324.0 +1.7/-3.3 days period that we attribute to a giant planet with a minimum mass of 2.83 +- 0.24 Mjup in an eccentric orbit with e=0.46 +- 0.04. A long-term quadratic drift, that we assign to be probably of stellar origin, is superimposed to the Keplerian solution.

THE CLUSTERED NATURE OF STAR FORMATION. PRE-MAIN-SEQUENCE CLUSTERS IN THE STAR-FORMING REGION NGC 602/N90 IN THE SMALL MAGELLANIC CLOUD*

ABSTRACT Located at the tip of the wing of the Small Magellanic Cloud (SMC), the star-forming region NGC 602/N90 is characterized by the H ii nebular ring N90 and the young cluster of pre-main-sequence (PMS) and early-type main-sequence stars NGC 602, located in the central area of the ring. We present a thorough cluster analysis of the stellar sample identified with Hubble Space Telescope/Advanced Camera for Surveys in the region. We show that apart from the central cluster low-mass PMS stars are congregated in 13 additional small, compact sub-clusters at the periphery of NGC 602, identified in terms of their higher stellar density with respect to the average background density derived from star counts. We find that the spatial distribution of the PMS stars is bimodal, with an unusually large fraction (∼60%) of the total population being clustered, while the remaining is diffusely distributed in the intercluster area, covering the whole central part of the region. From the corresponding color–magnitude diagrams we disentangle an age difference of ∼2.5 Myr between NGC 602 and the compact sub-clusters, which appear younger, on the basis of comparison of the brighter PMS stars with evolutionary models, which we accurately calculated for the metal abundance of the SMC. The diffuse PMS population appears to host stars as old as those in NGC 602. Almost all detected PMS sub-clusters appear to be centrally concentrated. When the complete PMS stellar sample, including both clustered and diffused stars, is considered in our cluster analysis, it appears as a single centrally concentrated stellar agglomeration, covering the whole central area of the region. Considering also the hot massive stars of the system, we find evidence that this agglomeration is hierarchically structured. Based on our findings, we propose a scenario according to which the region NGC 602/N90 experiences an active clustered star formation for the last ∼5 Myr. The central cluster NGC 602 was formed first and rapidly started dissolving into its immediate ambient environment, possibly ejecting also massive stars found away from its center. Star formation continued in sub-clusters of a larger stellar agglomeration, introducing an age spread of the order of 2.5 Myr among the PMS populations.

THE CLUSTER OF BLUE STARS SURROUNDING THE M31 NUCLEAR BLACK HOLE

We obtained U_330 and B band images of the M31 nucleus using the High Resolution Camera of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The spatial resolution in the U_330-band, 0.03" FWHM, or 0.1 pc at M31, is sufficient to resolve the outskirts of the compact cluster (P3) of UV-bright stars surrounding the M31 black hole. The center of the cluster is marked by an extended source that is both brighter and redder than the other point sources within P3; it is likely to be a blend of several bright stars. We hypothesize that it marks the location of the M31 black hole. Both stellar photometry and a surface brightness fluctuation analysis, show that the P3 stellar population is consistent with early-type main sequence stars formed in a ~100 - ~200 Myr old starburst population. Evolutionary tracks of post early asymptotic giant-branch stars, associated with late-stage evolution of an old population, also traverse the U and U-B domain occupied by the P3 stars; but we argue that only a few stars could be accounted for that way. PEAGB evolution is very rapid, and there is no progenitor population of red giants associated with P3. The result that P3 comprises young stars is consistent with inferences from earlier HST observations of the integrated light of the cluster. Like the Milky Way, M31 harbors a black hole closely surrounded by apparently young stars.

SOPHIE velocimetry ofKeplertransit candidates

The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called `Doppler boosting') in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252+/-0.025 Msun, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b.

Deep infrared imaging of close companions to austral A- and F-type stars

The search for substellar companions around stars with different masses along the main sequence is critical to understanding the different processes leading to the formation of low-mass stars, brown dwarfs, and planets. In particular, the existence of a large population of low-mass stars and brown dwarfs physically bound to early-type main-sequence stars could imply that the massive planets recently imaged at wide separations (10–100 AU) around A-type stars are disc-born objects in the low-mass tail of the binary distribution and are thus formed via gravitational instability rather than by core accretion. Our aim is to characterize the environment of early-type main-sequence stars by detecting substellar companions between 10 and 500 AU. The sample stars are also surveyed with radial velocimetry, providing a way to determine the impact of the imaged companions on planets at ≲10 AU. High-contrast and high angular resolution near-infrared images of a sample of 38 southern A- and F-type stars were obtained between 2005 and 2009 with the instruments NaCo on the Very Large Telescope and PUEO on the Canada-France-Hawaii Telescope. Direct and saturated imaging were used in the J to Ks bands to probe the faint circumstellar environments with contrasts of ~5 × 10-2 to 10-4 at separations of and 1″, respectively. Using coronagraphic imaging, we achieved contrasts between 10-5 and 10-6 at separations >5″̣ Multi-epoch observations were performed to distinguish comoving companions from background contaminants. This survey is sensitive to companions of A and F stars in the brown dwarf to low-mass star mass regime. About 41 companion candidates were imaged around 23 stars. Follow-up observations for 83% of these stars allowed us to identify a large number of background contaminants. We report detection of 7 low-mass stars with masses between 0.1 and 0.8 M⊙ in 6 multiple systems: the discovery of an M2 companion around the A5V star HD 14943 and detection of HD 41742B around the F4V star HD 41742 in a quadruple system. We resolve the known companion of the F6.5V star HD 49095 as a short-period binary system composed of 2 M/L dwarfs. We also resolve the companions to the astrometric binaries ι Crt (F6.5V) and 26 Oph (F3V), and identify an M3/M4 companion to the F4V star o Gru, associated with an X-ray source. The global multiplicity fraction measured in our sample of A and F stars is ≥ 16%. This has a probable impact on the radial velocity measurements performed on the sample stars.

Planets and brown dwarfs around A–F main-sequence stars: performances of radial-velocity surveys with HARPS and first detections

Due to their high v sin i , early-type main-sequence stars have been considered as unsuitable targets for radial-velocity surveys. Knowing about the presence of planets around those stars is however a key issue to understand the impact of the mass of the central star on the way planets form and evolve. We developped in recent years a method that allows to search for planets around those stars and started radial-velocity surveys focused on these targets. We report here some results obtained so far in terms of performances and detection.

Extrasolar planets and brown dwarfs around A–F type stars

(abridged) In the frame of the search for extrasolar planets and brown dwarfs around early-type main-sequence stars, we present the results obtained on the early F-type star Theta Cygni. Elodie and Sophie at OHP were used to obtain the spectra. Our dedicated radial-velocity measurement method was used to monitor the star's radial velocities over five years. We also use complementary, high angular resolution and high-contrast images taken with PUEO at CFHT. We show that Theta Cygni radial velocities are quasi-periodically variable, with a ~150-day period. These variations are not due to the ~0.35-Msun stellar companion that we detected in imaging at more than 46 AU from the star. The absence of correlation between the bisector velocity span variations and the radial velocity variations for this 7 km/s vsini star, as well as other criteria indicate that the observed radial velocity variations are not due to stellar spots. The observed amplitude of the bisector velocity span variations also seems to rule out stellar pulsations. However, we observe a peak in the bisector velocity span periodogram at the same period as the one found in the radial velocity periodogram, which indicates a probable link between these radial velocity variations and the low amplitude lineshape variations which are of stellar origin. Long-period variations are not expected from this type of star to our knowledge. If a stellar origin (hence of new type) was to be confirmed for these long-period radial velocity variations, this would have several consequences on the search for planets around main-sequence stars, both in terms of observational strategy and data analysis. An alternative explanation for these variable radial velocities is the presence of at least one planet of a few Jupiter masses orbiting at less than 1 AU. (abridged)

Stellar oscillations in planet-hosting giant stars

Recently a number of giant extrasolar planets have been discovered around giant stars. These discoveries are important because many of these giant stars have intermediate masses in the range 1.2-3 M⊙. Early-type main sequence stars of this mass range have been avoided by radial velocity planet search surveys due the difficulty of getting the requisite radial velocity precision needed for planet discoveries. Thus, giant stars can tell us about planet formation for stars more massive than the sun. However, the determination of stellar masses for giant stars is difficult due to the fact that evolutionary tracks for stars covering a wide range of masses converge to the same region of the H-R diagram. We report here on stellar oscillations in three planet-hosting giant stars: HD 13189, β Gem, and ι Dra. Precise stellar radial velocity measurements for these stars show variations whose periods and amplitudes are consistent with solar-like p-mode oscillations. The implied stellar masses for these objects based on the characteristics of the stellar oscillations are consistent with the predictions of stellar isochrones. An investigation of stellar oscillations in planet hosting giant stars offers us the possibility of getting an independent determination of the stellar mass for these objects which is of crucial importance for extrasolar planet studies.

Non-LTE modelling of the He I 10830 Å line in early-type main sequence stars

The near-IR 10 830 Å transition is a highly sensitive diagnostic for non-LTE effects in astrophysical plasmas. So far, non-LTE line-formation computations have failed to quantitatively reproduce observations of this line in the entire range of early-A to late-O main sequence stars. It is shown that the non-LTE modelling was insufficient, for the most part, either because of inaccurate photoionization cross-sections for the 2s 3S state or because of neglecting line blocking. New calculations based on state-of-the-art atomic data give excellent agreement with observation for the 10 830 Å feature, while profiles of the lines in the visual are retained.