Context. Among intermediate-mass and massive stars, Be stars are the fastest rotators in the main sequence (MS) and, as such, these stars are a cornerstone to validate models of structure and ...evolution of rotating stars. Several phenomena, however, induce under- or overestimations either of their apparent V sini, or true velocity V. Aims. In the present contribution we aim at obtaining distributions of true rotational velocities corrected for systematic effects induced by the rapid rotation itself, macroturbulent velocities, and binarity. Methods. We study a set of 233 Be stars by assuming they have inclination angles distributed at random. We critically discuss the methods of Cranmer and Lucy-Richardson, which enable us to transform a distribution of projected velocities into another distribution of true rotational velocities, where the gravitational darkening effect on the V sini parameter is considered in different ways. We conclude that iterative algorithm by Lucy-Richardson responds at best to the purposes of the present work, but it requires a thorough determination of the stellar fundamental parameters. Results. We conclude that once the mode of ratios of the true velocities of Be stars attains the value V/V sub(c)Asymp totically = to 0.77 in the main-sequence (MS) evolutionary phase, it remains unchanged up to the end of the MS lifespan. The statistical corrections found on the distribution of ratios V/V sub(c) for overestimations of V sini, due to macroturbulent motions and binarity, produce a shift of this distribution toward lower values of V/V sub(c) when Be stars in all MS evolutionary stages are considered together. The mode of the final distribution obtained is at V/V sub(c)Asymp totically = to 0.65. This distribution has a nearly symmetric distribution and shows that the Be phenomenon is characterized by a wide range of true velocity ratios 0.3 <, ~ V/V sub(c)<, ~ 0.95. It thus suggests that the probability that Be stars are critical rotators is extremely low. Conclusions. The corrections attempted in the present work represent an initial step to infer indications about the nature of the Be-star surface rotation that will be studied in the second paper of this series.
Context.
The ‘Great Dimming’ of the prototypical red supergiant Betelgeuse, which occurred between December 2019 and April 2020, gives us unprecedented insight into the processes occurring on the ...stellar surface and in the inner wind of this type of star. In particular it may bring further understanding of their dust nucleation and mass-loss processes.
Aims.
Here, we present and analyse VLTI/MATISSE observations in the N band (8–13 µm) taken near the brightness minimum in order to assess the status of the dusty circumstellar environment.
Methods.
We explored the compatibility of a dust clump obscuring the star with our mid-infrared interferometric observations using continuum 3D radiative transfer modelling, and probed the effect of adding multiple clumps close to the star on the observables. We also tested the viability of a large cool spot on the stellar surface without dust present in the ambient medium.
Results.
Using the visibility data, we derived a uniform disk diameter of 59.02 ± 0.64 mas in the spectral range 8–8.75 µm. We find that both the dust clump and the cool spot models are compatible with the data. Further to this, we note that the extinction and emission of our localised dust clump in the line of sight of the star directly compensate for each other, making the clump undetectable in the spectral energy distribution and visibilities. The lack of infrared brightening during the Great Dimming therefore does not exclude extinction due to a dust clump as one of the possible mechanisms. The visibilities can be reproduced by a spherical wind with dust condensing at 13 stellar radii and a dust mass-loss rate of (2.1–4.9) × 10
−10
M
⊙
yr
−1
; however, in order to reproduce the complexity of the observed closure phases, additional surface features or dust clumps would be needed.
Context.
The surface brightness – color relationship (SBCR) is a poweful tool for determining the angular diameter of stars from photometry. It was for instance used to derive the distance of ...eclipsing binaries in the Large Magellanic Cloud (LMC), which led to its distance determination with an accuracy of 1%.
Aims.
We calibrate the SBCR for red giant stars in the 2.1 ≤
V
−
K
≤ 2.5 color range using homogeneous VEGA/CHARA interferometric data secured in the visible domain, and compare it to the relation based on infrared interferometric observations, which were used to derive the distance to the LMC.
Methods.
Observations of eight G–K giants were obtained with the VEGA/CHARA instrument. The derived limb-darkened angular diameters were combined with a homogeneous set of infrared magnitudes in order to constrain the SBCR.
Results.
The average precision we obtain on the limb-darkened angular diameters of the eight stars in our sample is 2.4%. For the four stars in common observed by both VEGA/CHARA and PIONIER/VLTI, we find a 1
σ
agreement for the angular diameters. The SBCR we obtain in the visible has a dispersion of 0.04 magnitude and is consistent with the one derived in the infrared (0.018 magnitude).
Conclusions.
The consistency of the infrared and visible angular diameters and SBCR reinforces the result of 1% precision and accuracy recently achieved on the distance of the LMC using the eclipsing-binary technique. It also indicates that it is possible to combine interferometric observations at different wavelengths when the SBCR is calibrated.
Aims.
We present a detailed visible and near-infrared spectro-interferometric analysis of the Be-shell star
o
Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations.
Methods.
We ...analyzed spectro-interferometric data in the H
α
(VEGA) and Br
γ
(AMBER) lines using models of increasing complexity: simple geometric models, kinematic models, and radiative transfer models computed with the 3D non-LTE code HDUST.
Results.
We measured the stellar radius of
o
Aquarii in the visible with a precision of 8%: 4.0 ± 0.3
R
⊙
. We constrained the circumstellar disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The emitting disk sizes in the H
α
and Br
γ
lines were found to be similar, at ~10–12 stellar diameters, which is uncommon since most results for Be stars indicate a larger extension in H
α
than in Br
γ
. We found that the inclination angle
i
derived from H
α
is significantly lower (~15°) than the one derived from Br
γ
:
i
~ 61.2° and 75.9°, respectively. While the two lines originate from a similar region of the disk, the disk kinematics were found to be near to the Keplerian rotation (i.e.,
β
= −0.5) in Br
γ
(
β
~ −0.43), but not in H
α
(
β
~ −0.30). After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the circumstellar disk in both lines: a base disk surface density Σ
0
= 0.12 g cm
−2
and a radial density law exponent
m
= 3.0. The same kind of discrepancy, as with the kinematic model, is found in the determination of
i
using the BeAtlas grid. The stellar rotational rate was found to be very close (~96%) to the critical value. Despite being derived purely from the fit to interferometric data, our best-fit HDUST model provides a very reasonable match to non-interferometric observables of
o
Aquarii: the observed spectral energy distribution, H
α
and Br
γ
line profiles, and polarimetric quantities. Finally, our analysis of multi-epoch H
α
profiles and imaging polarimetry indicates that the disk structure has been (globally) stable for at least 20 yr.
Conclusions.
Looking at the visible continuum and Br
γ
emission line only,
o
Aquarii fits in the global scheme of Be stars and their circumstellar disk: a (nearly) Keplerian rotating disk well described by the viscous decretion disk (VDD) model. However, the data in the H
α
line shows a substantially different picture that cannot fully be understood using the current generation of physical models of Be star disks. The Be star
o
Aquarii presents a stable disk (close to the steady-state), but, as in previous analyses, the measured
m
is lower than the standard value in the VDD model for the steady-state regime (
m
= 3.5). This suggests that some assumptions of this model should be reconsidered. Also, such long-term disk stability could be understood in terms of the high rotational rate that we measured for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.
Context. Accretion and outflow processes are of fundamental importance for our understanding of the formation of stars and planetary systems. To trace these processes, diagnostic spectral lines such ...as the Brγ 2.166 μm line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Aims. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the Brγ line emission with mass infall (magnetospheric accretion, gaseous inner disks) or mass outflow processes (stellar winds, X-winds, or disk winds). Methods. Using the VLTI/AMBER instrument, we spatially and spectrally (λ$/\Delta\lambda$ = 1500) resolved the inner ($\la$5 AU) environment of five Herbig Ae/Be stars (HD 163296, HD 104237, HD 98922, MWC 297, V921 Sco) in the Brγ emission line as well as in the adjacent continuum. From the measured wavelength-dependent visibilities, we derive the characteristic size of the continuum and Brγ line-emitting region. Additional information is provided by the closure phase, which we could measure both in the continuum wavelength regime (for four objects) as well as in the spectrally resolved Brγ emission line (for one object). The spectro-interferometric data is supplemented by archival and new VLT/ISAAC spectroscopy. Results. For all objects (except MWC 297), we measure an increase of visibility within the Brγ emission line, indicating that the Brγ-emitting region in these objects is more compact than the dust sublimation radius. For HD 98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD 163296, HD 104237, MWC 297, and V921 Sco we identify an extended stellar wind or a disk wind as the most likely line-emitting mechanism. Since the stars in our sample cover a wide range of stellar parameters, we also search for general trends and find that the size of the Brγ-emitting region does not seem to depend on the basic stellar parameters (such as the stellar luminosity), but correlates with spectroscopic properties, in particular with the Hα line profile shape. Conclusions. By performing the first high-resolution spectro-interferometric survey on Herbig Ae/Be stars, we find evidence for at least two distinct Brγ line-formation mechanisms. Most significant, stars with a P-Cygni Hα line profile and a high mass-accretion rate seem to show particularly compact Brγ-emitting regions ($R_{{\rm Br}\gamma}/R_{{\rm cont}}$ < 0.2), while stars with a double-peaked or single-peaked Hα-line profile show a significantly more extended Brγ-emitting region (0.6 $\la$ $R_{{\rm Br}\gamma}/R_{{\rm cont}}$ $\la$ 1.4), possibly tracing a stellar wind or a disk wind.
Context. Stellar rotation is a key in our understanding of both mass-loss and evolution of intermediate and massive stars. It can lead to anisotropic mass-loss in the form of radiative wind or an ...excretion disk. Aims. We wished to spatially resolve the photosphere and gaseous environment of 51 Oph, a peculiar star with a very high vsini of 267 km s-1 and an evolutionary status that remains unsettled. It has been classified by different authors as a Herbig, a β Pic, or a classical Be star. Methods. We used the VEGA visible beam combiner installed on the CHARA array that reaches a submilliarcsecond resolution. Observation were centered on the Hα emission line. Results. We derived, for the first time, the extension and flattening of 51 Oph photosphere. We found a major axis of θeq = 8.08 ± 0.70 R⊙ and a minor axis of θpol = 5.66 ± 0.23 R⊙. This high photosphere distortion shows that the star is rotating close to its critical velocity. Finally, using spectro-interferometric measurements in the Hα line, we constrained the circumstellar environment geometry and kinematics and showed that the emission is produced in a 5.2 ± 2 R⋆ disk in Keplerian rotation. Conclusions. From the visible point of view, 51 Oph presents all the features of a classical Be star: near critical-rotation and double-peaked Hα line in emission produced in a gaseous disk in Keplerian rotation. However, this does not explain the presence of dust as seen in the mid-infrared and millimeter spectra, and the evolutionary status of 51 Oph remains unsettled.
Context. The star HD 87643, exhibiting the “Be phenomenon”, has one of the most extreme infrared excesses for this object class. It harbours a large amount of both hot and cold dust, and is ...surrounded by an extended reflection nebula. Aims. One of our major goals was to investigate the presence of a companion in HD87643. In addition, the presence of close dusty material was tested through a combination of multi-wavelength high spatial resolution observations. Methods. We observed HD 87643 with high spatial resolution techniques, using the near-IR AMBER/VLTI interferometer with baselines ranging from 60 m to 130 m and the mid-IR MIDI/VLTI interferometer with baselines ranging from 25 m to 65 m. These observations are complemented by NACO/VLT adaptive-optics-corrected images in the K and L-bands, and ESO-2.2m optical Wide-Field Imager large-scale images in the B, V and R-bands. Results. We report the direct detection of a companion to HD 87643 by means of image synthesis using the AMBER/VLTI instrument. The presence of the companion is confirmed by the MIDI and NACO data, although with a lower confidence. The companion is separated by ~34 mas with a roughly north-south orientation. The period must be large (several tens of years) and hence the orbital parameters are not determined yet. Binarity with high eccentricity might be the key to interpreting the extreme characteristics of this system, namely a dusty circumstellar envelope around the primary, a compact dust nebulosity around the binary system and a complex extended nebula suggesting past violent ejections.
Context. The eclipsing binary method for determining distance in the local group is based on the surface brightness-color relation (SBCR), and early-type stars are preferred targets because of their ...intrinsic brightness. However, this type of star exhibits wind, mass-loss, pulsation, and rotation, which may generate bias on the angular diameter determination. An accurate calibration of the SBCR relation thus requires careful analysis. Aims. In this paper we aim to quantify the impact of stellar rotation on the SBCR when the calibration of the relation is based on interferometric measurements of angular diameters. Methods. Six stars with V − K color indices ranging between –1 and 0.5 were modeled using the code for high angular resolution of rotating objects in nature (CHARRON) with various rotational velocities (0, 25, 50, 75, and 95% of the critical rotational velocity) and inclination (0, 25, 50, 75, and 90 degrees). All these models have their equatorial axis aligned in an east-west orientation in the sky. We then simulated interferometric observations of these theoretical stars using three representative sets of the CHARA baseline configurations. The simulated data were then interpreted as if the stars were non-rotating to determine an angular diameter and estimate the surface-brightness relation. The V − K color of the rotating star was calculated directly from the CHARRON code. This provides an estimate of the intrinsic dispersion of the SBCR relation when the rotation effects of flattening and gravity darkening are not considered in the analysis of interferometric data. Results. We find a clear relation between the rotational velocity and (1) the shift in zero point (Δa0) of the SBCR (compared to the static relation) and (2) its dispersion (σ). When considering stars rotating at less than 50% of their critical velocity, Δa0 and σ have about 0.01 mag, while these quantities can reach 0.08 and 0.04 mag, respectively, when the rotation is larger than 75% of the critical velocity. Besides this, the inclination angle mostly has an impact on the V − K color: i< 50° (resp. i> 50°) makes the star redder (resp. bluer). When considering the 150 models, Δa0 and σ have 0.03 and 0.04 mag, respectively. These values are slightly but not significantly modified (about 0.03 and 0.01 mag in Δa0 and σ, respectively) when considering different CHARA configurations. Interestingly, these 150 models, regardless of the interferometric configuration, are consistent with the empirical SBCR, which is within its dispersion of 0.16 mag. In addition, if one only considers projected rotational velocity Vrotsini lower than 100 km s-1, then Δa0 and σ have 0.02 and 0.03 mag, respectively. Conclusions. To calibrate the SBCR interferometrically at the 0.02 mag precision (or lower), one should consider (1) a baseline configuration covering all directions of the (u, v) plan; (2) a sample of stars with rotational velocity lower than 50% of their critical velocity or, alternatively, stars with Vrotsini lower than 100 km s-1; (3) homogeneous visible and infrared photometry precisely at the 0.02 mag level or lower.
Context. Unveiling the structure of the disks around intermediate-mass pre-main-sequence stars (Herbig Ae/Be stars) is essential for our understanding of the star and planet formation process. In ...particular, models predict that in the innermost AU around the star, the dust disk forms a “puffed-up” inner rim, which should result in a strongly asymmetric brightness distribution for disks seen under intermediate inclination. Aims. Our aim is to constrain the sub-AU geometry of the inner disk around the Herbig Ae star R CrA and search for the predicted asymmetries. Methods. Using the VLTI/AMBER long-baseline interferometer, we obtained 24 near-infrared (H- and K-band) spectro-interferometric observations on R CrA. Observing with three telescopes in a linear array configuration, each data set samples three equally spaced points in the visibility function, providing direct information about the radial intensity profile. In addition, the observations cover a wide position angle range (~97°), also probing the position angle dependence of the source brightness distribution. Results. In the derived visibility function, we detect the signatures of an extended (Gaussian FWHM ~ 25 mas) and a compact component (Gaussian FWHM ~ 5.8 mas), with the compact component contributing about two-thirds of the total flux (both in H- and K-band). The brightness distribution is highly asymmetric, as indicated by the strong closure phases (up to ~40°) and the detected position angle dependence of the visibilities and closure phases. To interpret these asymmetries, we employ various geometric as well as physical models, including a binary model, a skewed ring model, and a puffed-up inner rim model with a vertical or curved rim shape. For the binary and vertical rim model, no acceptable fits could be obtained. On the other hand, the skewed ring model and the curved puffed-up inner rim model allow us to simultaneously reproduce the measured visibilities and closure phases. From these models we derive the location of the dust sublimation radius (~0.4 AU), the disk inclination angle (~35°), and a north-south disk orientation (PA ~ 180-190°). Our curved puffed-up rim model can reproduce reasonably well the interferometric observables and the SED and suggests a luminosity of ~29 $L_{\odot}$ and the presence of relatively large ($\ga$1.2 μm) Silicate dust grains. Our study also reveals discrepancies between the measured interferometric observables and the puffed-up inner rim models, providing important constraints for future refinements of these theoretical models. Perpendicular to the disk, two bow shock-like structures appear in the associated reflection nebula NGC 6729, suggesting that the detected sub-AU size disk is the driving engine of a large-scale outflow. Conclusions. Detecting, for the first time, strong non-localized asymmetries in the inner regions of a Herbig Ae disk, our study supports the existence of a puffed-up inner rim in YSO disks.
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