To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure firstname.lastname@example.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
From November 2019 to April 2020, the prototypical red supergiant Betelgeuse experienced an unexpected and historic dimming. This event was observed worldwide by astrophysicists, and also by the general public with the naked eye. We present here the results of our observing campaign with ESO’s VLT and VLTI in the visible and infrared domains. The observations with VLT/SPHERE-ZIMPOL, VLT/SPHERE-IRDIS, VLTI/GRAVITY and VLTI/MATISSE provide spatially resolved diagnostics of this event. Using PHOENIX atmosphere models and RADMC3D dust radiative transfer simulations, we built a consistent model reproducing the images and the photometry.
We observed the circumstellar environments (CSEs) of the semiregular AGB stars L2 Puppis, R Doradus and EP Aquarii with ALMA. (1) The molecular emission in the L2 Pup nebula reveals an edge-on rotating disk. (2) PV diagrams of the 28SiO emission in the inner CSE of R Dor expose a pattern pointing to an inclined rotating disk. (3) The CO emission in the CSE of EP Aqr reveals a nearly face-on equatorial density enhancement (EDE). The inner EDE strongly resembles theoretical wind-Roche-lobe-overflow models. The SiO emission points to a potential companion. The combination of (1), (2) and (3) suggests that a link may exist between the type of AGB pulsations and the morphological nature of the CSE.
The mass of a Cepheid is a fundamental parameter for studying the pulsation and evolution of intermediate-mass stars. But determining this variable has been a long-standing problem for decades. Detecting the companions (by spectroscopy or imaging) is a difficult task because of the brightness of the Cepheids and the close orbit of the components. So most of the Cepheid masses are derived using stellar evolution or pulsation modeling, but they differ by 10-20 %. Measurements of dynamical masses offer the unique opportunity to make progress in resolving this mass discrepancy.
The first problem in studying binary Cepheids is the high contrast between the components for wavelengths longer than 0.5 μm, which make them single-line spectroscopic binaries. In addition, the close orbit of the companions (<40 mas) prevents us from spatially resolving the systems with a single-dish 8m-class telescope. A technique able to reach high spatial resolution and high-dynamic range is long-baseline interferometry. We have started a long-term program that aims at detecting, monitoring and characterizing physical parameters of the Cepheid companions. The GAIA parallaxes will enable us to combine interferometry with single-line velocities to provide unique dynamical mass measurements of Cepheids.
Massive evolved stars contribute to the chemical enrichment of the Galaxy. When they die as supernova but also through their mass loss during the several thousands of years of their red supergiant (RSG) phase. Unfortunately the mass loss mechanism remains poorly understood. Detailed study of the CSE and photosphere of nearby RSGs is required to constrain this scenario.
Betelgeuse is the closest RSG (197 pc) and therefore has a large apparent diameter (~ 42 mas) which makes it a very interesting target. For several years, our team has lead a multi-wavelength and multi-scale observing program to characterize its mass loss. We will review here our recent results in near-infrared interferometry.
The very high spatial resolution provided by current interferometers (VLTI, CHARA, …) makes it possible to measure directly the change in angular diameter of more than thirty Cepheids over their pulsation cycle. When combined with radial velocity measurements, this allows us to measure precisely their distances in a quasi-geometrical way. This is an essential information to calibrate the Cepheid's Period-Luminosity law. On the other hand, a careful analysis of the dynamical structure of their atmosphere is required to avoid biases in their distance determination.
Optical interferometry is the only technique giving access to milli-arcsecond (mas) resolution at infrared wavelengths. For Cepheids, this is a powerful and unique tool to detect the orbiting companions and the circumstellar envelopes (CSE). CSEs are interesting because they might be used to trace the Cepheid evolution history, and more particularly they could impact the distance scale. Cepheids belonging to binary systems offer an unique opportunity to make progress in resolving the Cepheid mass discrepancy. The combination of spectroscopic and interferometric measurements will allow us to derive the orbital elements, distances, and dynamical masses. Here we focus on recent results using 2- to 6-telescopes beam combiners for the Cepheids X Sgr, T Mon and V1334 Cyg.
Cepheids are one of the most famous standard candles used to calibrate the Galactic distance scale. However, it is fundamental to develop and test independent tools to measure their distances, in order to reach a better calibration of their period-luminosity (P-L) relationship. We present here the first results obtained with the Integrated Parallax of Pulsation (IPoP) method, an extension of the classical Baade-Wesselink method that derives the distance by making a global modelisation of all the available data. With this method we aim to reach a 2% accuracy on distance measurements.
Cepheid masses are also an essential key for our comprehension of those objects. We briefly present an original approach to derive observational constraint on Cepheid masses. Unfortunately, it does not lead to promising results.
The violent convective motions, low surface gravity, and high brightness of red supergiants combine to trigger an intense stellar wind. As the distance from the star increases, the standard scenario is that the ejected material forms molecules, then dust particles. But this general picture is still fragmentary. Our goal is to assemble a better understanding of mass loss in Betelgeuse, considered as a prototype for its class, from its photosphere to the interface of its wind with the interstellar medium. Thanks to its proximity ( ≈ 197 pc), it is ideally suited for such a detailed study. Over the past few years, our team obtained an extensive set of observations of Betelgeuse from high angular resolution instruments, probing a broad range of spatial scales: 1) interferometric imaging of its photosphere and close envelope in the near- and thermal-IR domains (IOTA/IONIC), 2) adaptive optics “lucky imaging” of its compact molecular envelope (VLT/NACO, 1.0–2.2 μm), and 3) diffraction-limited imaging of its dusty envelope (VLT/VISIR, 8–20 μm). From our interferometric data, we detect the presence of spots at the surface of the star, as well as CO and H2O molecules, and dust particles close to the star. Within 6 R⋆, the flux distribution of the envelope is compatible with the presence of the CN molecule. At a few arcseconds from the central star, we observe a complex dusty envelope probably containing O-rich dust (e.g. silicates, alumina). We present an overview of these recent observational results and ongoing work. They provide new hints on the physical and chemical mechanisms through which Betelgeuse interacts with its environment.
Classical Cepheid stars have been considered since more than a century as reliable tools to estimate distances in the universe thanks to their Period-Luminosity (P-L) relationship. Moreover, they are also powerful astrophysical laboratories, providing fundamental clues for studying the pulsation and evolution of intermediate-mass stars. When in binary systems, we can investigate the age and evolution of the Cepheid, estimate the mass and distance, and constrain theoretical models. However, most of the companions are located too close to the Cepheid (∼1–40 mas) to be spatially resolved with a 10-meter class telescope. The only way to spatially resolve such systems is to use long-baseline interferometry. Recently, we have started a unique and long-term interferometric program that aims at detecting and characterizing physical parameters of the Cepheid companions, with as main objectives the determination of accurate masses and geometric distances.
We present the results of the analysis of our recent interferometric observations of
Betelgeuse, using the AMBER instrument of the VLTI. Using the medium spectral resolution
mode (R ~ 1500) we detected the presence of the water vapour and
carbon monoxide (CO) molecules in the H and K bands. We also derived the photospheric
angular diameter in the continuum. By analysing the depth of the molecular lines and the
interferometric visibilities, we derived the column densities of the molecules, as well as
the temperature and the size of the corresponding regions in the atmosphere of Betelgeuse
(the MOLsphere) using a single shell model around the photosphere. Our results confirm the
findings by Perrin et al. (2004)
and Ohnaka et al. (2011) that the
H2O and CO molecules are distributed around Betelgeuse in a MOLsphere
extending to approximately 1.3 times the star’s photospheric radius.
The bright southern star δ Vel is a multiple system comprising at least three stars, with two of these stars in an eclipsing binary system. We searched for infrared excess and determined fundamental stellar parameters for the three components from a combination of photometry, spectroscopy, adaptive optics imaging (VLT/NACO), thermal infrared imaging (VLT/VISIR) and near-infrared interfero- metry (VLTI/AMBER). The main eclipsing component is a pair of A-type stars in rapid rotation. We modeled the photometric and radial velocity measurements of the eclipsing pair Aa-Ab using a self consistent method based on physical parameters (mass, radius, luminosity, rotational velocity). From this modeling, we derive the fundamental parameters of the eclipsing stars with a typical accuracy of 1%. We find that they have similar masses, respectively 2.43 ± 0.02 and 2.27 ± 0.02 M⊙. As we spatially resolve the eclisping binary orbit, we also derive the parallax of the system, 39.8 ± 0.4 mas, in satisfactory agreement (− 1.2σ) with the Hipparcos value (40.5 ± 0.4 mas). Finally, we measured the differential astrometric displacement of the center of light of the eclipsing binary relatively to δ Vel B with an uncertainty of ≈100 microarcseconds per epoch, from adaptive optics imaging, thus demonstrating the capabilities of this technique for high-precision astrometry. This accuracy appears to be limited by astrophysical noise (activity of the Aa or Ab stars).
Improvement of the calibration of the Cepheid period–luminosity relation (Leavitt's Law) is one of the main challenges to improve the accuracy of the Hubble constant, H0. Many parallax-of-pulsation methods are promising but have not yet delivered sufficiently accurate distances: observational biases, such as the projection factor, still dominate. We propose a global parallax-of-pulsation method, combining all observables (photometry, spectroscopy, and interferometry), to (i) reduce statistical errors, (ii) use the redundancy among observables to validate our approach, and (iii) achieve 2% accuracy for individual Cepheid distances.
We have started a survey of M 33 in order to find variable stars and
Cepheids in particular. We have obtained more than 30 epochs of
g'r'i' data with the CFHT and the
one-square-degree camera MegaCam. We present first results from this
survey, including the search for variable objects and a basic
characterization of the various groups of variable stars.
Behaviour of classical Cepheids is summarized from the observational point of view. The paper concentrates on trends in recent observational studies of, and important empirical results obtained on Cepheids.
We give a brief overview of Cepheids and of their modeling, with particular
emphasis on F/O1 Beat Cepheids.
Then we revisit the use of Period Ratio vs. Period diagram
(Petersen diagram) for fundamental/first overtone Beat Cepheids, because
they allow one to put very tight constraints on their metallicity Z.
The Petersen diagram is shown to be largely independent of the helium content Y,
of the mass-luminosity relation that is
used in their construction, and of stellar rotation rates.
However, it shows sensitivity to the
chemical makeup of the elements that are lumped into the metallicity
parameter Z. The Petersen diagram for the new Asplund, M., Grevesse, N., & Sauval, A.J. (2005, ASP Conf. Ser., 336, 25) solar mix is
compared to that for for the older, “standard” solar mix of Grevesse, N., & Noels, A., (1993).
Until recently, Cepheids were considered implicitely as "nak- ed" stars, i.e. devoid from circumstellar material. In 2005, VINCI/ VLTI and FLUOR/CHARA interferometric observations revealed circumstellar envelopes (hereafter CSEs) around Cepheids. Their presence may have an impact on the determination of distances, and could bear testimony of their mass-loss history. Although their observation is made difficult by the brightness of the Cepheids themselves, many observation techniques have the potential to improve our knowledge of their physical properties. We discuss in particular long-baseline interferometry, visible and infrared imaging, spectroscopy and radio observations.
We present a new numerical scheme describing the convective velocity field using
two parallel radial columns in order to represent up- and downstream flows.
The two columns are coupled by horizontal exchange via fluid flow and radiation over their common interface.
For this geometrical setup, the equations of radiation hydrodynamics are solved time dependently using
an implicit scheme which has the advantage of allowing very large time steps.
Finally we demonstrate our approach for the example of a Cepheids convection zone.
A nonlinear radiative pulsating model for the long-periodic
Cepheid l Carinae is presented. The LTE metallic lines and non-LTE
hydroen lines are calculated and compared with the HARPS observations.
We found that the weak and medium metallic lines well correspond to
the radial Cepheid pulsation, while the Hα and some other
strong lines reveal very strange behaviour which is not consistent with
usual Cepheid pulsation. We conclude that l Car must have an
extended circumstellar envelope, and discuss its possible parameters.
As a by-product of β Cephei and δ Cephei radiative model surveys we have found interesting cases of multimode pulsation. In case of β Cephei stars, we found two multimode domains with two or three modes being involved. The origin of the multimode pulsation can be traced to one of the two different mechanisms: either to the non-resonant coupling of the two excited modes (double-mode pulsation) or to the 2ω1$\simeq$ω0 + ω2 parametric resonance (triple-mode pulsation). In case of δ Cephei models, a triple-mode pulsation domain, connected with the 2:1 resonance,
2ω0$\simeq$ω2, is found. These models are of theoretical interest only, since they do not obey a proper mass-luminosity relation.